Can someone (politely) explain to me why this is actually a bad thing? Since afaik without knowing any details of "the plot" how is it possible to say that this is good or bad? [...]
1.) I don't want to be required to play the game multiple times to experience all the companions. Especially since, even if I did do 3-4 playthroughs to get all 9-12 companions, I'd still
likely miss out on cool interactions between companions that I never had together in the same playthrough. I play rpgs for the story, gameplay, and companion interactions/quests. Locking out the majority of companions each playthrough severely hurts one of those 3 pillars.
2.) D&D is a class based system, and party locking prevents changing up my party to combat different threats. If I don't have SH in the party, then am I just locked out of a cleric companion for the rest of the game? If my party doesn't include the 1-2 companions with thieves' tools proficiency (let alone expertise), then lock-picking is made incredibly more difficult for the rest of the game. Am I supposed to heavily nerf one of my other companions, spending one of my very-limited ASIs/feats to gain said tool proficiency? Hiring mercenaries is a lame solution to this because you can't interact with mercenaries; see point #1.
3.) Finally, Larian's history works against them. The only data point we have is DOS2, where there wasn't a reason for the rest of the companions to be killed off. They just were, because Larian said so. Barring any comment otherwise from Larian, it's reasonable to be worried that they'll do a similar thing for BG3.