The difference between this situation and the Braixen/Delphox situation being that Braixen was more popular than Delphox, so it wouldn't make sense to evolve Braixen into the less popular form. Brionne is not more popular than Primarina, so it doesn't make sense to leave it at the less popular form. Unless the writers want to pull a Unova and just give it to them to prevent Lana from getting it, but that would just be a dick move.At least I won't have to suffer seeing Primarina on the main cast, those guys will be back and show it off. It's the Braixen situation.
This feels closer to the Sanpei situation IMO, where Lana meets another character who happens to have the evolved form of her starter and both act as mentors. Sanpei debuting Greninja didn't prevent Ash from getting one. In this case, it would be no different for Lana.
That being said, the more pessimistic side of me thinks you may be right simply because it's easier to do that, and the writers won't have to focus on Lana as much. It would basically be screwing over hers and Popplio's development because there's not much left for either of them if Popplio doesn't evolve, but just because it doesn't make sense doesn't mean it won't happen.