Further, having simple class mechanics is not "a weird mess" it is what people wanted and what I believe they still want.
I didn't say it was so that's kind of an elaborate tangent to go off on about a thing that isn't present lol jeez.
Wizard and Barbarian both have a simpler core class than a fighter does.
Only if you pretend spells don't exist. Wizards are infinitely more complex to build than Fighters because of spell selection alone. It's not even close. Barbarian is about the same. Cleric and Rogue are similar. That's it. Monk for example has drastically more totally unnecessarily shoved into the core chassis.
I did not say the Champion was "vital", but it is the most widely played fighter and in fact the most widely played subclass of all.
It's barely the former and definitely not the latter.
The last datapoint is from 2020, over two years ago, and Battlemaster was 1% behind. And a much earlier survey had Champion much further ahead, so we can say that Champion is likely rapidly losing ground to other subclasses. I have zero doubt it's behind Battlemaster by now - indeed it would take a bizarre reversal of trend for that to be true.
It's definitely not the most popular subclass, either. The same Beyond statistics which put it as "most widely played" (again by 1% over Battlemaster) put Warlock ahead of Fighter in popularity (13% Warlock, 12% Fighter), and Hexblade as by far the most popular Warlock subclass at 38%. Draconic Bloodline Sorcerer is also more popular than Champion, even though only 10% of players play Sorcerer, 28% play Draconic Bloodline.
But at the end of the day Champion is good enough to be the top dog in terms of popularity and that makes an argument that it needs to be changed pretty weak overall.
Except as I've demonstrated, it isn't. And Hexblade should never be changed, by that logic.
But I'm not necessarily arguing it needs to change, but rather if you object to a more complex default, all classes need chassis as simple as Fighters, and to have as much put into subclasses.