I don't have a very high opinion of Larian but even I don't think your theory is very likely, just because they still have to come up with new plot stuff for them both, create in-game models and quests, etc. They also have to now worry about getting them somewhat right so as not to trigger fan outrage. Plus they got Matt Mercer for the game, which doesn't seem like an easy thing to do late in the game. I would argue that creating a new companion would actually be easier since they don't have to worry about any past threads to deal with, and no expectations. And honestly, I could come up with three ideas for a good-aligned companion right now, it's not that hard and they still have to do the work for plot stuff. Plus I feel like Larian's style would lend itself more to the opposite, they'd start out with the iconic characters and then change their mind and think they could do better.
