No. There are many more axies of class diversity than complexity.
That may be, but complexity (which I use only because it's the word most people use; I personally prefer "mechanical engagement") is one of the single most important axes of differentiation, and--as we saw with 4e--putting everyone on exactly the same resource schedule makes people mad and makes people think they're "all samey" even if they aren't.
No, but there are players who strongly prefer simple characters, and those players should not be barred from certain classes. All classes should be able to be simple.
Then all classes have to be trapped in simplicity.
That's the cost of doing that. Making it so every class is simple leaves
insufficient room for adding worthwhile complexity.
Because that cuts those classes off from players who want or need a simple character.
And that cuts off people who want classes that are inherently complex--as I would prefer to say it,
classes that require high mechanical engagement--because you cannot serve two masters. You cannot have a class that really actually makes the complexity
sing, that really makes the mechanical engagement
worth doing, when it's a mere bolt-on. I wish that weren't true, but I've seen the results time and time and time again. Trying to bolt-on complexity after the fact is either beyond the limits of designers in general to produce, or every professional designer I've ever followed has been unable to achieve it, despite it being somehow easy to do. The latter seems, to me, rather unlikely, given my experiences. Perhaps your experience differs, but if so you're gonna have to show at least one example where it did, in fact, achieve that result--because I've seen
way too many examples that did not.
IMO, this is one of the limitations you simply have to accept when you elect to design a class-based game. Some classes simply will not offer
everything that
every player wants out of them. They can offer everything that
some players want, or some particular thing for every player, but not everything for every player all of the time.