100% agree with the OP. I really do hope they'll reconsider. Some of my thoughts on the subject below.
I am by no means saying you should be able to drag along every single recruitable character regardless of circumstances. But the limitation should come from story reasons. And by that I mean organic, believable, realistic story reasons that make sense and that FEEL like they make sense. Not "rocks fall, everyone dies" story reasons that are handwaved with whatever excuse. Good reasons would include: antagonizing a companion, a companions strongly disagreeing with your actions, incompatible companions. That's already a lot of limitations. Arbitrary locking your party is just... frustrating and imo brings no benefit.
To add to that, what about the later acts? If you are only able to have those 3 companions, you have to walk on eggshells if you don't want to become companionless. You have to concentrate more on appeasing companions than on roleplaying your character. Mercenaries aren't really a solution to losing a whole companion storyline.
Regarding replayability: I am strongly of the opinion that replayability should come from EXPANDING options, not limiting them. That and BG3 seems to have ridiculous replayability regardless of party composition. Additionally, should a player want to limit the current playthrough to 3 specific companions, there is nothing in the way of that. (Apart from, perhaps, conflicts as mentioned above.) It doesn't work the other way around.
Another matter is that, imo, it would be really nice to be able to send specialized teams for different "missions". You'd certainly send a druid to deal with a nature spirit and a rogue to deal with criminal underground.
It's made even worse by the party size of 4. Which means you'll only get to know well 3 companions per playthrough. I intend to play the game until I'm sick of it (after full release), but can't see myself playing it, say, 12 times, at least not in the first few years after release. Let's assume there will be 9 companions in the finished game (conservative guess). That means you'll need 3 playthroughs just to learn all companion stories (if playing as a custom character). That doesn't account for all the other variables, which, we are told, are numerous. Even if one disregards other game elements and just considers companions - those 3 playthroughs give us all companion stories, yes, but there are so many more combinations. We're going to miss out on most party interactions. If I'm not mistaken, there are 84 party combinations for 9 companions and 3 party slots. Of course, not every combination will matter and most interactions will likely be between two characters, not all three. But it's still a lot to miss out on, not just in one playthrough, but in multiple. In BG1&2, there were many interesting party interactions, and while few people would see all of them, a lot would see most of them. There were some interactions between three characters as well: like Xan intervening in certain instances of party-breaking conflicts. It might be different for a short 10-hour game intended for once-per-month replaying, but BG3 is a fully-sized RPG. I'm not saying any given player should see every secret and interaction (obscure, hidden "that's so cool!" stuff is really nice), but if most players are to miss most of the (companion-related) content despite playing several times? Not sure about that.
Perhaps I'm exaggerating a bit, but I do believe the current approach to party comp (as it appears to work after Act I) is problematic to say the least.
