If I had to guess, it's fear that this game will do well enough that TPC will decide that the core audience isn't worth pandering to and all future games will be more primarily casual experiences.
You can say 'well they already made the casual game, now they can make the core game' but that isn't how sales works. Corporate follows the money, and if the money is in nostalgia cash-out, super casual, low effort and low content experiences then that's almost certainly going to influence the 'core' games.