having more open classes leads to huge open design space.
Issue is that having more open classes isn't the same thing has having more classes, and thats where you're closing off the design space.
The dissatisfaction with those that came after the original 3/4 is rooted in bad, if not nonexistent, design. Like how the Ranger to this day is still just a cobbled together homebrew mishmash rather than a bespoke design. (With the only attempt to do bespoke being so poorly executed now people are biased against it)
Classes should matter if they exist, and if they don't matter they shouldn't be there.
And fwiw, I personally think a hybrid approach is best. My own game has classes that are designed as such, but multiclassing is so permissive that in practice it acts as a point buy type system. Between 20 classes and 80 subclasses and the ability to mix and match all of them, including taking multiple subclasses, while still supporting Classes that actually matter, you do a lot better than going one way or the other.
Those who don't want to care about their build so much won't have to, and those that do will be given a more substantial playground to do so than anything else on the market. (Given theres Skill Paths on top of this as well as another big dimension to character building)
Like, there is no niche protection
Niche protection is stupid and an unnecessary constraint when you want the game to be permissive towards people playing what they want to play, and not what the game needs them to play.
And its especially stupid because its a cooperative game and you shouldn't be coming into it from that kind of selfish perspective in the first place. You're not lesser if someone else can do the same thing.
Which is why I personally advocate for moving a lot of class features into narratively achieved progression. Have your main combat schtick and your numbers go up with level, your horizontal progression and your utility abilities should be achieved in play and open to any character.
That tends to lead into the Prestige Class problem where people are eventually going to skip the required plot beats just so they can play what they want to play.
Its one of those things where one has to be conscious of how people are playing the game and embrace it. Probably the wisest design decision from WOTC that they haven't repeated that mistake in two editions.