TL;DR: It's not about classless system being better than one with classes or vice versa. Multiclassing combines disadvantages of both with no significant gain.
I don't agree with the italicized part, but the rest of your post was excellent.
I never understand why many DM's feel the need to dictate arbitrary restrictions on rules as written in the core books...
The next part of your sentence actually answers the implied question:
...since the core rules are the foundation for everyone's shared hopes and expectations for the game experience.
We all develop our own idea about what D&D is about and how it is meant to be played (we may have more than one of these). However, every few years these people we pay to make official products seem to want to change the texts that these assumptions grew out of it. So new people coming to the game are going to pick up a book and start having their impressionable minds imprinted with a version of what D&D is about that may not be the same as other players who started with a different set of text to read.
I play more RPGs than just D&D. And I enjoy an extremely broad amount of D&D stuff itself. Still, there are certain things that are and are not D&D done right to me. I just have no interest in playing D&D in other ways--there are too many cool things I want to do with D&D that can't be done as well with other games, and there are too many other games that can do other stuff better than D&D.
When I'm spending my time being the DM, we are playing my version of D&D, because it is literally of no interest to me not to. If my friends want to play something else, then we'll play something non-D&D. If one of them wants to run D&D in a way I find not worth the time investment, I'll skip it.
The 5e playtest was great at figuring out what most people want out of D&D, and I was a very active participant and feel they did a really good job overall. However, that doesn't mean that I agree with every decision. Because I don't agree with everything in the core rule books, and they aren't all 'true' in my games, it is important for me to let potential players know about those--preferably before they get their heart set on an unapproved character concept.
It is
because the core rulebooks set the standard for what new players expect out of D&D that many of us are so passionate about what content such books are going to contain, and is why we have these sorts of problems when editions keep changing. Fortunately 5e is here for a while, so those who started with it will likely get a longer run than those who started with 3e or 4e.
I have my share of house rules, but I present each one to the group, discuss why I think its an improvement and only put it into play if my players agree its an improvement for all - or at least worth testing.
That's a great way of doing it. However, you are taking an equal group investment approach. I do the same thing if my group is brainstorming up something to do; we'll throw around ideas and everyone who wants to gets equal input. Once we get to a point where someone is volunteering to be the GM, that person is going to get more input, because they are doing a whole heck of a lot more work, and that better be work they want to do. So in our recent superhero game that we've been playing across multiple systems for rules comparisons, we're doing just that. When your turn as GM is up, we still have discussion before we recreate our characters for the new system, but GM gets final say on stuff.
Now, that's assuming an equal group investment campaign design. This is by no means the only way we play our RPGs. Often, one particular member of our group has a campaign they want to run and they ask if anyone is interested. In that case it is their way or the highway. Sure, there may be some negotiation. But it is the GM's game, and if you don't like it you skip that game; no hard feelings. It would be an inappropriate imposition to expect the GM to change his vision of an RPG experience he wants to present, and if a player's character concept isn't appropriate for his scenario, that is exactly what they are doing.
I want to say I don't know why people have a problem with GMs acting as such, but I have enough knowledge of psychology that I do know. I would just encourage people to compare playing an RPG to other forms of non-democratic recreation they enjoy before having a kneejerk reaction that in order for a game to enjoyable for them every participant has to have full and equal input on how it's going to go down.
