Gaskets suck from a manufacturing and service standpoint. They also fail at much higher rates than something like 3M 4200 (which is probably something similar to what Apple uses). Apple can encase the battery like they did on old phones, but it will be at the expense of thickness / battery life. On top of all of that, the "glue" provides shock resistance, and adheres the battery to the screen which distributes stress loads (ie stiffness, dropping).
If I were guessing Apple will go with a weaker glue that can be reused / removed with a scraper combined with some sort of screws. They could also go with an enclosed battery pack with a large gasket and screws and is sold only in the EU (it would have a smaller battery).
We will never know, but if I were guessing screen breakage will go up and water claims will increase.