I would argue that DnD5e is very lax with party composition.
Yeah, but as already argued in the past, it's not really a matter of NECESSITY as much of enjoying the variety.
Four slots feels incredibly restricting (borderline "suffocating") because once you pick for one of two roles you WANT, it leaves you with basically no freedom of how to fill the remaining two if you enjoy diversifying.
Eeh, I can't say this problem hit me. In single player, I can always swap my party in the rare occasions I want to diversify. Is there a stealth mission? Astarion is in. Do I need a better front line for this one? Laezel is there. More healing? Shadowheart it is, and so on. Hirelings will most likely guarantee that you will be able to pick any class and any race at any time in a playthrough. The few multiplayer games I made, we just imposed ourselves not to pick duplicate classes and it worked fine every time. But to be fair multiplayer is basically a different game. I can see your point making sense at higher difficulty since that often forces players to pick a very optimal composition, therefore forcing you to have key classes and spells and leaving very little room to creativity. We will see once it gets introduced.