I also think the small amount of companions is a much bigger problem than 4 vs. 6 party members.
It's a combination of BOTH, plus the impending threat of wiping out all the non-grouped companions after Act 1 (if they intend to stick with that idea after two years of extremely negative feedback about it).
It also adds a great deal of replayability when you can have different party compositions. It feels like in BG3 I will always be stuck with the same companions even after we get 3 more. You should have a choice for an Arcane spellcaster, a Cleric or a Rogue. Having to take Gale and Shadowheart on every single playthrough likely means there will be no new playthrough.
That's an issue that was already pointed several times across the months in the countless threads about the topic (and consequently in the Mega-thread, too): the "Few slots mean very diverse playthrough" is more often than not a complete myth.
Most players will always consider some particularly key role too crucial to give up on them, which in practical terms narrows down the "room for variety of party composition" even more.
And before someone jumps the gun, this doesn't mean you COULD NOT play in their absence. You can pretty much go through the game however you want, even solo. Just that many will not want to.
