The comments about this decision ranges from "I don't really mind it" to "no way this is stupid as shit".
I honestly don't know what are Larian motives for this decision. While it might have worked for DOS2 because of the competitive nature of the game (the idea that all the main characters are competing each other on a cosmic level) and had the potential to be really great, Larian seriously massed that up. So even if the attempt was a big failure, at least there was a great idea there. Or so I thought.
But it seems like Larian has different motives for this idea of commitment to a party, because they do again for Baldur's Gate. Weather they can find a compelling narrative reason for it remains to be seen, but at least it's clear now that they are doing it because of design philosophy, not because the narrative demands it. And it's very bad news for the story and characters.
It means they are forcing their writers to find ways to make all possible companions unavailable for no good reason other than this vague "commitment" concept.
To be honest this one really pisses me off
Repeating something they did from DO2 is not "design philosophy" its them turning themselves into a one-trick pony. Worst case scenario they felt they did the idea wrong in Do2, and felt the poor response was due to mishandling and are saying to themselves "we will do it right this time"
Instead of coming to the more obvious conclusion "this is a thing most games dont do... for a good reason"
And in this game, a game about a group of disparate people who are from vastly different backgrounds and existences, who under different circumstances would NEVER be friends... having to come together, and learn to work together to solve the same problem and situation they ave all been thrust into... is extremely undermined if the party is broken up for any reason.
Breaking the party up after Act 1, and forcing the player to commit to one party... is not mechanically advisable, it is not narratively advisable, it is not good for the image of there company... it is a bad idea.