I don't know why you think we'll be stuck with the 3 companions we escape from. They might just be in our party, while the others make their own party and escape and we find them later on.
Well, I don't know for sure, but isn't that supposed how MP works? I mean if every companion can at every time be taken over by a real person by drop-in MP isn't that companion automatically always the same person.
Just think about following situation. A group of four real persons start a campaign in DOS 2 together: each of those is creating their own character. Then, after some time one of the four has no time to play and the rest of them continue with an "AI bot companion". Do you really think it's possible that the people are then able to dispose that character then, replacing him by a new, random/predefined one? I guess the missing player will be delighted to find out that the character he created is gone... Same scenario with one player starting alone though, having his game open for drop-in MP.
That's also the reason why I'm so heavily opposed to the idea of drop-in/drop-out MP, even if you want to play the game completely alone, from beginning to end. The game is still riffled by the same basic design no matter what, at least in DOS 1. The only thing Larian changed for that was that they enabled predefined traits for the "AI bot" that was called your "2nd main character" at release. And all they plan for DOS 2 is to replace these predefined traits with a fully fledged out origin story aka backstory. The rest will very likely just work the same like in DOS 1, now with four PCs that can bcome AI bots if you play alone (which doesn't make them real companions in any strech of the traditional sense of the word in traditional CRPG design).
So maybe I'm all wrong but then again I'd be very curious to know how SP was supposed to work, given the still existing possibility of drop-in/drop-out MP. With the existing basic design of DOS 1 multiple companions for DOS 2 is imo pretty much impossible...