I think a more interesting dynamic is already in place. If you choose to do certain things in the game, you can lose Astarion. Other things, you can lose Wyll. I believe Shadowheart will disappear too if you make certain decsions.
Rather than "arbitrarily lock the party", they could just do more to resolve the characters. I know there is a reputation system already in place, this is an easy goto method to get rid of characters by making them all conflict in interests... so eventually it will be inevitable and the player decides proggressively who stays and who goes by being good or bad (which works only if there is a recrutiment function later in the game that lets players subsidize any class they want with a generic NPC so if they lose all their Origin characters somehow they can just make all generics)
