Interesting read, you've clearly put a lot of thought into this and you've expressed your view very convincingly. It's always fun to discuss these things, here's a few points I'd like to respond to:
About 5e, I personally believe that it has been designed with the precise intentions of being a different game on each and every table. One of the most discussed topics about 5e is "what houserule do you use?", and other than that we can see how many subsystem the OpenGL generated, each one of them modifying here and there 5e base system bringing it in a new setting, adding rules and removing them.
That's what I would call the "optimistic" outlook. The other way to look at this is that 5e players feel the constant need to homebrew new mechanics, rules and content because RAW 5e simply doesn't provide them when it should. You have to play game-designer in order to arbitrate even the simplest scenarios like a player wanting to buy a spell scroll (should it cost 500gp or 5000gp?) or craft an adamantine sword. Praising DnD5e for being open to modifications would be the same as judging Skyrim by how it feels to play after you've modded it to perfection - it means accepting lazy and shoddy design because you can fix it yourself. But here's the thing - I don't believe DnD is
that open to homebrewing in the first place. Here's why.
The guidelines for building creatures, features or items in 5e are typically lackluster or straight-up nonexistent. This means that whenever I homebrew, I must rely strictly on my own intuitive understanding of the system and its tenuous balance. Whenever I introduced homebrew into my game I felt like I was doing so on a wing and a prayer, never being sure that it's fair, that it's balanced, that it won't break something or ruin somebody's experience 10 sessions down the line. I used to think this is normal, that it's the only way things can be and that I have only my own ineptitude as a gm to blame. Then I tried a system with properly designed rules.
In Pf2e the ruleset provides a rock-solid foundation for you in common situations and edge-case scenarios alike. The math behind the game becomes clear and intuitive as you familiarize yourself with it and is supported by tons of guidelines and helpful tables. Making new homebrew content thus becomes a joyful, no-stress creative process with little to no guesswork. Thanks to the sheer breadth of available options I need to spend less time homebrewing new rules to accommodate the specific roleplay ideas the players come up with. And when I need to homebrew something after all, thanks to the rigidity of the rules and - I want to stress this part - FUNCTIONAL GUIDELINES, I spend significantly less time biting my nails over balance. All of that makes for a LOT of saved time which I can instead use for the things that actually matter - being creative, preparing the story, content or props. Ironically, I feel like a more solid ruleset gives me
more freedom to make the game suit the needs of my group, not less.
Ok, this is interesting, but I'm gonna provoke you. Has the setting ever mattered in the DnD system? I honestly know only 3.x and 5e, so I can't talk about the others editions, but I feel like the answer is not. D&D is already a setting agnostic system, a system that can pretty much work wherever you place it. Some settings have their own rules that goes on top of D&D itself, but the base system remains there. As always, this is not bad, quite the opposite, it's something needed if you want a system that works both in eberron and raveloft as much as faerun, but I think it can answer your question.
Absolute blasphemy
but jokes aside, DnD is NOT a setting-agnostic system at all, it's just that the settings you named (except Symbaroum, which I'm unfamiliar with) are all DnD settings. You can't take the classes, races and mechanics of DnD and put them into Dragon Age, Game of Thrones or even Lord of the Rings without significant compromise. A tabaxi has no place in Middle Earth, the differences between wizards, sorcerers or clerics make no sense in Dragon Age and the whole thing would make no sense in Westeros. The classes in particular have tons of lore behind them and that lore doesn't mesh with just any setting, not even within the high fantasy bracket.
The only way one could consider DnD a setting-agnostic system is by not considering classes, races and items a part of the system and only seeing them as a kind of 'content'. The core mechanics (e.g. roll a d20 with this and that kind of bonus to see if you succeed) could work for any setting, but surely that's not all the system is.
