I just finished Wasteland 3 which adds new companion choices all the way through the game, even up to around 75% of the game being finished. By that halfway point, I've already settled into combat strategies for my companions and it's hard to integrate a new one as a replacement. Especially if they're auto-leveled to match the current party, which may be a mix of skills and attributes you wouldn't choose for them.
The companions in Baldur's Gate III don't have their skills and spells automatically set for them when they go up in level to match the party's level.
Again using Wasteland 3 as an example, and other games work like this too -- you sometimes get a side-quest as you move through the game that's designed around one of the companions. You ideally want that one in your party to get the full dialog and best outcome. This can mean re-hiring one you've dismissed just for that quest, then dismissing them again afterwards.
This feels incredibly "gamey." If we have to commit to the party at the end of Act 1, Larian can just block off all the quests for companions you're not using. We'll never see those awkward notices about "be sure to have Companion XXX in your party" when receiving a new quest notice. Any companion quests will be smoothly integrated for just your current party.
Quests designed around a companion shouldn't be a problem as long as there are alternate resolutions to those quests, and there's no need to give notices telling you that you should bring a certain companion with you (although, it'd be helpful to mention the companion in the quests area of the journal).
I'm not sure why it matters if the extra events and best outcome for a quest aren't available without a specific companion.
As for adding a companion just for one quest, fast traveling to get a companion from camp wouldn't be as difficult as finding them again, which is what you have to do in some games, and if you don't want to go get them, you wouldn't have to.
The quests wouldn't be available at all if they were blocked the way that you're suggesting.
There's really no good reason to not let players choose different companions later.
I'm fine with the party being set in stone, somewhat as if it was an actual tabletop campaign (with players you can count on). It will be a difficult choice but I don't mind it at all.
The difference with video games is that what the party members do and say is also set in stone.
