I honestly see Larian more or less recycling the DOS2 trope.
Those you don't pick are turned to mindflayers and you will fight them toward the end of the game. It seems obvious to me, which is reductive.
Whatever some people may claim, Larian's statement during original Q&A strongly indicated that we will have too chose a more permanent party.
I don't think they will turn into mindflayers - while that's the original set up, it just doesn't seem to be what those modified tadpoles are designed to do. Getting mindcontrolled by Absolute, however, seems like a very real possibility - it would happen to PCs party already in EA if not for Gith McGuffin (McGithin? - you are welcome Larian narrative team). My bet is that at some point those not in the party (and therefore not under Weapons protection) will succumb to the influence and only present party will remain themselves. Unless, Larian took notice of the negative feedback and will walk back on that decision.
Edit: To me, personally, it is not a biggy - I generally tended to create one party in BG1&2 and stick to it, so after having whole act to decide whom I want to take, I am fine will letting the rest go. I found D:OS2 approach refreshing, even if narratively they didn't manage to get much milage out of it - it was done very abruptly and arbitrarily, and with unsatisfying payoff.
