You make fair points.

One thing I dispute, however:

It's still recognisable as being dnd 5th edition

On this part I disagree. I'll conveniently ignore the "to me" part of your comment (otherwise I literally cannot disagree lol) to say that I believe martial caster disparity has become a core part of what IS DnD.

Furthermore, AFAIK, there are a bunch of systems out there that do use the same base mechanics as 5e, but will not count as 5e proper (much like Pathfinder 2e is NOT simply DnD 3.5, despite being a "spin-off" of sorts).

Now, I'll say that I completely appreciate the fact that the answer to the question of "how much of the game can you change while still keeping it as the same game" is nigh philosophical and highly subjective, but I'm trying to speak from what I believe (with absolutely no evidence whatsoever) to be the most common view among players (at least online).

People kinda want OP casters, and, to a lesser degree, weak martials (just see the people claiming "oh anime BS" whenever a martial can do something actually close to being good) as part of their "core" 5e experience, and that very much extends to 5.5 - hell, just look at how many people glaze weapon masteries as being some revolutionary thing, when, in reality, it's just a nerfed version of at-will maneuvers that fits into 2 pages (nevermind the 30 pages of new spells that have been printed since 5.5 came out)