Your "cowardice" is my "change for the sake of change is rarely a good thing".
A better explanation is that they are doing the best they can with the expertise and knowledge they have, backed up by playtesting and surveys, to make the game work for as many people as possible. Like all human endeavors it will never be perfect.
I just don't buy it. You don't design classes like the 5E Bard, the 5E Warlock, the 5E Paladin and so on, all modern and interesting designs, then suddenly do a 180 and revert to 3E-style design for a few classes for no clear reason (I should note that Warlock and Paladin weren't even classes I liked before 5E - I was actually kind of sighing that Warlock was included at all until I saw the design). Likewise the Fighter and Rogue, whilst perhaps not ideal, are significantly modernized, as is the Barbarian. The Cleric and Wizard are more or less "as expected".
And it's no accident 3 out of the 4 "reverted to 3E" classes are widely (and accurately) regarded as some of the least effective classes and all four as the least well-designed classes in 5E, either - presumably because they didn't get the attention and thought put into them that other 5E classes did get. Sorcerer that's particularly obvious - they were clearly thinking down one direction, then suddenly reverted, despite, as far as literally anyone can tell, feedback being strongly positive on the DND Next Sorcerer (AFAICT they've never explained this, but maybe I've forgotten). Did the Monk/Druid/Ranger get DND Next playtests at all? I forget at this point.
Calling the 5E Sorcerer's design "change for change's sake" is absolutely as insulting as anything I'm saying, too - it certainly didn't look like change for change's sake - it looked carefully considered.
Just because the choices they make don't happen to be the ones you personally want, it's insulting to use phrases like "cowardice". It comes off as ... I guess petty is the best word I can come up with.
That's obviously untrue and unfair if you actually read my posts, and I know you do. Loads of 5E design "isn't the choices I personally want" - like 30% of it! So trying to make a case that I'm calling "cowardice" just because of that is obviously and patently false, and rather disappointing, frankly. I don't think, for example, the HD-based healing system is the result of a panic about going too far, nor do I think the plain-jane design of the Wizard is, nor do I think the almost excessively generous default Long Rest system nor excessively penalized Short Rest system is the result of anything like that.
I'm talking about one very specific and unusual thing that happened - there are three "plausible" causes I can see:
1) They got scared of going too far with redesigns and reverted to 3E approaches without really considering how they would fit into 5E - this explains why they're all clumsier designs too.
2) Someone at WotC put their foot down and just overruled everything else (other designers, surveys, etc.) - this seems to have happened occasionally, so it's possibility.
3) They ran out of time, and reverted simply because they didn't have time to design new/better mechanics for these classes - it's a little hard to explain the Sorcerer with this, but we know from WotC comments that they did basically run out of time on the design of both 4E and 5E, and it's possible this was the cause. It tallies with a lot of other design issues in 5E, particularly the dubious quality and confused-seeming DMG (relative to other D&D DMGs, almost certainly including the 2024 DMG, which I expect to be pretty great if I'm honest), so I can't rule it out.
Some combination of all three could also be true - i.e. they saw they were running out of time, had some potential options for how to deal with it, and decided or were told that reverting to 3E-style designs was the best way to use the time left to them before it had to go to print.
