That someone better.
IMO 5e has very nearly no serious issues. Plenty of minor stuff, but nothing that threatens to make the game not work.
I agree that if they do 50th/6e D&D, they will change as little as possible.
But I think that there is design space to improve things while still maintaining backwards compatibility to adventures, monsters and most class features.
Combat could easily be made faster. The action economy tweaked. Skills streamlined or backgrounds/careers as skills like barbarians of lemuria. Inspiration dropped and the spend for advantage effect added to what Hit Dice do, (just one meta-mechanic guys...) Drop the whole badly implemented Traits, Ideals, Bonds, and Flaws stuff that most groups ignore. Bring back morale rules for monsters. And the exhaustion rules need a bit of a revamp... etc..
There are things that can be done to actually improve the game, streamline mechanics and enhance speed of play at the table.
You are probably right in the direction a potential 50th/6e D&D will go: Virtually zero mechanical change. Just sourcebook changes moved to core.
I just think it would a shame to waste 10 years worth of real-world playtesting to not improve the game where you can, and still maintain a very high degree of backwards compatibility.