If in 5e everyone can do anything, why do we even have classes?
I think it is great in BG3! The classes are quite different to play, and you have the flexibility to take the companions you want based on personality, alignment, and/or which combat style(s) you like!
But none of this answers my question. And btw, my question was entirely sincere and serious. If I were forced to pick only one thing that says this is what defines D&D and makes it distinct from other similar high-fantasy RPGs, I would say that one thing is D&D's class system. For me, without many distinct classes, D&D is no longer D&D.
If in 5e everyone can do anything, why do we even have classes?
I don’t think they can, or if so then I’ve not found a way to achieve it! But there seem to be multiple ways to cover the functions that an adventuring party needs that are interesting to discover. I find each class still feels distinctive, though, even if they’re not as restrictive as the ones I’m used to from games based on earlier D&D versions.
So then you agree that having different CLASSES of companions in your party composition matters, and saying 'party composition in 5e doesn't matter' is false.