Party locking is a bad idea. However, I have no problem with the death,etc of the act 1 companions if if fits the storyline and that other non origin characters can be recruited. Not just mercenaries, but fully fleshed out characters….
Party members leaving because of your choices, that is fine... or even because of there choices (heroic sacrifices are cool for example
Having a "dramatic plot twist" where "all your party dies or becomes mind flayers" and you are stuck with three part members... thats not good