This might be another sticking point for both sides of the discussion. You mention 'DM Fiat' here, and a number of other posters have also talked about it... how removing 'DM Fiat' is better for the game in their opinion. As though the Dungeon Master not making choices for what the NPCs end up doing and instead relying on the results of dice is better than the alternative. I certainly can understand that desire. It removes (or at least lessens) the possibility of a "bad DM" making bad choices or unfair choices. Removing 'DM Fiat' turns the game much more into a game, and thus players who do not trust the skills or attitude of their DM can still play and expect relatively "fair" results.
Do not force a false dichotomy here. The world is not comprised solely of "absolutely wonderful DMs" and "just dirt worst DMs." There's a spectrum, and it turns out that an awful lot of DMs are just...mediocre. Not great, without somehow lacking moral fiber: well-meaning but inept or bumbling; brilliant writers, but terrible designers; absolute beasts at combat design, but incapable of keeping lore consistent from one session to the next; awesome planners, but terrible improvisers; friendly and approachable, but far too fickle (again, not a moral failing, just too easily led by whim or presentation); etc., etc. There is no end to the possible ways someone can be a mixed-bag DM or a just-kinda-okay DM or a great-except-for-one-Persian-flaw or whatever else.
We neither need nor want rules to "save" us from bad-faith DMing; in all likelihood, no such rules exist. But it is
useful to have rules which steer all of the above DMs—imperfect, erring,
human—away from preventable mistakes and errors. And guess what? Rules are really good for that exact thing. They don't help keep any man, honest or dishonest, stuck to honesty; they help keep
imperfect men away from easy mistakes, or, in the best cases, help guide those men to even greater success.
So don't make this a false dichotomy, where we must choose between no rules at all so as to not hobble anyone, or draconian and likely useless rules with the false premise of making good men out of bad. Recognize that this comes from a desire to make
effective DMs out of imperfect ones—something that, when undertaken with moderation and care, is good for everyone.
My response to that would be that I genuinely feel really bad for any players out there who play with DMs for whom their instincts and attitudes are not trustworthy enough to garner a good game. That must suck as a player that the "fiat" their DM employs is not worthwhile.
What if it isn't a matter of
trust, but a matter of recognizing human flaws? Of knowing that humans all too easily fall to bias and error, particularly when it comes to difficult things like statistics, or perverse incentives, or accidental degenerate solutions?
I would say though that in a normal case scenario... your prototypical DM should, could, and would make logical, sound, reasonable decisions in reaction to what the players do, such that elaborate mechanical systems wouldn't be necessary to "protect" the players.
Forget about protection then. You should most certainly know by now that lots of people absolutely don't make "logical, sound, reasonable decisions," not out of moral failings, but because people are not logic engines. We do foolish things for strange reasons. We fail to properly communicate all the damn time. We allow emotion, or bias, or false beliefs, or any of a million other things to cloud our reasoning and preclude doing the things that would be most beneficial for us or others or both.
I know for me... the DMs I play with are all good DMs for whom if they make a reactive decision in response to what I say... that I trust their reaction to be reasonable for whatever the NPC is that the DM is playing. And if they make that reaction with or without a die roll... that's perfectly acceptable to me.
Would you then say you have never ever had a merely flawed DM? A merely mediocre one? One that could have incredible skill in one aspect of DMing but be, charitably,
not the best in other areas?
Because if you've never had a DM with even a single flaw, then yes, I would absolutely call you
insanely lucky.
But in my estimation the more hopeful response would be to just teach DMs how to make better calls and thus become more trustworthy than take away their agency and replacing it with mechanical systems.
And what of systems which do not do that, but instead
enhance their agency?
Have you established that every rule necessarily destroys DM agency? Is it not possible that some, being useful tools rather than albatrosses around the neck, actually help the DM do
more than they could achieve on their own?
Because that road will eventually lead to CRPGs, where you don't need a human DM anymore because every response is generated mechanically. (And yes, I know the "slippery slope" argument is almost always extremely weak and fully admit that it is weak here too... but I use it merely as illustration for what removing DM agency and replacing it strictly with mechanics starts to feel like, if not actually become.)
Yeah, sorry, this is a textbook slippery slope argument and is exactly as weak as you say. It may be harsh, but your illustration of the feeling would have been far better served with something not so...well. Flawed. Emotional. Driven by things other than being "logical, sound, [and] reasonable." Which seems like a good way of showing how a person can be pushed toward things
they themselves recognize as non-logical.
Rules—good ones, well-made, showing restraint in where and how they are made and used—help us to fight back against human imperfections such as these. They'll never make such imperfections go away. Nothing will. But the fact you cannot make a problem 100% go away is
not a reason to choose to do nothing about it.