In general, content bloat leads to quality dilution. If the options offered are clustered too close together you have redundant material and easy comparisons as to which is the stronger or weaker choice. If the options spread out too far then it becomes hard to keep things on-theme and within the intended power range. And in both cases, more options means more possible permutations, which leads to choice paralysis in the casual players and extreme minmaxing by the character optimization crowd.What is the downside to "unnecessary number of (sub)classes"? What is the bad thing that happens when there are more options?
I wouldn't necessarily agree that we're at the bloat point already, or that the 5e PHB has entire classes that could be cut. Well, maybe the Sorcerer. I'm not unsympathetic to the argument that it has the weakest class role and identity. But in general, I think the slow roll of new material has been paced well.