Nobody here is insisting that curated games are the only way to have good games. Great games can be had with wide options. Great games can also be had with extremely limited options. Context, context, context (again). It depends on the game and the group. Surely you can't disagree with that?
I'd believe you if you had not just used examples that painted a curated game in a good light (Michelin Star Chef, Happily married man, ect) and a wide option game in a poor light ("mass audience" chef, playboy who "plays the field", ect)
So, yeah, I can agree that good games can be had either way. That is my point. However, your side seems to constantly make the comparison where your side is this higher tier of quality. Again, and again, and again. Oofta claimed that his way created deeper worlds with more integrated histories. Other people have claimed that too many options dilute the game, ruin themes, ect ect ect.
So, if people stopped making the comparisons to paint one side as superior to the other, I'd stop calling them out on it.
I don't see what's so hard to understand about it. Text is always mutable; that's the nature of text. It's subject to all manner of interpretation, emendation, addition, deletion, etc. Whether by the hand of a DM acting alone or a DM acting with the express consent and input of all players, the text can always be altered to suit the group, because it has no power over the people using it. Rules-as-written only have as much authority as the people implementing those rules choose to grant.
Rules-in-force, which is to say the actual game as it's played, are necessarily less mutable, because, well, the decisions have all been made. "These are the written rules we're using, those are the house-rules we're implementing, this is how the DM makes rulings in corner-cases, that is the set of precedents that inform rulings, etc." The rules-in-force cover everything from the finalization of those early decision-points (again, regardless of whether they're made by the DM alone or with player input) right up through the very present moment when the game is being played (under which circumstances it is nearly always the DM alone implementing and adjudicating rules, deciding "what happens" in the fiction as a response to player decisions, and for the most part acting in those exact circumstances as the one inalienable "authority" over the game).
And this "play-in-the-moment" is actually the least mutable thing of all, because you obviously can't change what's happening as it's happening without dissolving the whole game into a miasma of meaningless, chaotic discontinuity. Once a thing has happened in the game—once the DM has made a ruling—it has to stand in order for the game to have anything like causality and an arrow of time.
Now, here's the rub: setting lore is no different from game rules. Call one fluff and the other crunch if you like, but there's really no difference between them for the purposes of this discussion. A published setting guide has no more authority over my table than a published rulebook. But once I've made the decision—and again, you can say "I" meaning the DM or "we" meaning all the players here, and the argument doesn't change—it stands. It has to. If I've (we've) decided that I'm (we're) playing the 1983 edition of Basic D&D, but with a house-rule granting maximum hit points at 1st level, that's how my (our) game world works. If I (we) decide that I'm (we're) playing a setting with no elves and no tieflings, that's how my (our) game world works. Simple as that.
Next, understand that for some DMs, creating the setting (and writing mechanics and implementing house rules) very much is tantamount to finalizing those decisions that turn the mutable rules-as-written into the immutable rules-in-force. Hell, if the setting is persistent between campaigns, every aspect of creating the setting, deciding what happens in the campaign world between campaigns, advancing the fantasy world's timeline—that's gameplay, as surely as the 0.5 seconds of present moment that it takes a DM to adjudicate what happens when a player thrusts a 10'-pole into the nozzle of an oil-trap.
And that is how we go from changeable guidelines to adamant law. Who has the authority to make these final decisions will vary from group to group depending on the social contract, but for most groups by far, the longstanding traditional norm has been that this authority rests with the DM. It doesn't have to; it just often does.
You seem to have talked around the point I was making.
If we were to bring up that in the Forgotten Realms X, Y or Z is true, we would immediately be brought to task that all things are mutable and that the game empowers us to make any change we want.
But, when DM A comes to us with a setting where X, Y or Z is true... it is unmutable, unchangeable, and the very attempt to change it is an overreach of the players.
And the players should approach the rules as being infinitely mutable... by the DM, because what the DM says is going to be absolute, so they simply should ask what the real rules are.
So, you can see the disconnect I'm talking about, right? Actual Rules? Changeable. DMs rules? Completely unalterable.
Seems an odd position to take that the rules are completely changeable... until you as the DM tell us that it isn't.
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And you can't just put a idea aside for a better opportunity to use it?
Considering it can be as long as a year in-between character creations, and inspiration will hit again for that new game... you are essentially asking us to abandon the idea and never run it.
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I've played with dozens of DMs over the years and am at 100% has at least one thing changed, usually multiple things. The same with everyone I've talked to, though I'm sure now that I've said that the contrarians will pop saying, "Not me! Everyone I've played with goes 100% by the rules."
It could not easily be nothing. It's possible that it's nothing, but not easily nothing.
One thing? In a game set with hundreds of thousands of rules? (After all, lore is rules too) How... exactly like I said. The game would be mostly intact.
If I tell someone that there are no elves and the Bladesinger is now just another subclass like Evoker, and they ask "who," they weren't listening to me.
So, you aren't listening to me. You are just going to make your world more shallow by not replacing the lore you removed. "It just is, no one cares why"
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Agreed.
Cool concept. But, if the DM was clear, like I stated they are, then why go down that character path? Save it for another campaign. It's a good idea, but just doesn't fit that specific campaign. I have friends with hundreds of character ideas. I have twenty on the back-burner right now. It is no big deal to set one aside for the correct time and place. Heck, sometimes they morph into something better when they sit and stew in the thought process for awhile.
Because that isn't how my brain works.
I get excited about an idea, it develops in my head, I'm excited to work with those concepts... I can't just turn that off. I can't just say "Man, this is going to be so cool, and now I don't care because there is a new idea that is going to be equally cool."
And, in that particular instance I had the ideas cascade while I was helping another player create their character. I went down that path and had that character almost fully written, before finding out that the DM hadn't included Waukeen, and we had to talk about how or if to include her.
I don't know... I've known a few playboys that are much happier than married men.

I don't judge.
And I very clearly state both sides have fun and - neither one is "wrong."
No. You are making a judgement. A quality judgement. "Fun" is not a measure of I care about from your example, because it is beside the point for how you judged the two sides.
To this I can only explain it with these words:
That is where ONE of the stories are. It is a group game, not a solo adventure. The player in your Tolkien example literally has 100,000+ options at their disposal. Are you suggesting they can only have fun by playing an ent? That is the only story they can come up with? I am sorry, I don't get it.
The DM can only come up with one world? That is the
only world they can come up with, aren't there literally 100,000+ options at their disposal?
I'm sorry, I don't get it.
I think you are being a little narrow here. The DM creates the world. They have a range. They set this range based off the books D&D publishes and their own written material. Because one DM's range is a little more narrow than another's does not make them immutable or unchangeable. It means they are trying to follow their creation's parameters, whatever those may be.
And it is all guidelines, until the DM decides it isn't. Correct. They are the ones putting the work in 90% of the time. They are the ones spending money. They are the ones that the players are asking to run the game.
I mean, in a different style of game, or a table where DM's rotate every session, sure. You are correct. But a traditional D&D game has a DM. They decide things to help facilitate the campaign and encourage the story. They provide the boundaries. It is why the adventurers don't leave Rivendell and within a mile enter New York City. They set the boundary. But, I suppose in your game, if the player wanted to insist they were from New York City because they wanted to be Snake Plissken, you would say sure. Good on you. But, most DM's have boundaries that they set. Because you don't like the boundaries does not mean the DM is being hypocritical as you suggest.
Max was talking earlier that a player should enter the game with the expectation that any or every rule in the game has been changed. So, a player should enter the game with the expectation that every rule is mutable...
Except that they should never question the rules set down by the DM, or attempt to have those rules changed.
This is a perception problem, isn't it? The Players need to go in with the idea that anything could have been changed, but they also need to go in with the idea that the rules cannot be changed because they want them to be.
And, I'm a player here recently. I've definitely put in more money than my DM. I've put in a lot of work. The DM asked me to play, not the other way around. It is much closer to a collaboration.