Let's not forget that going on tour and having to play one-and-a-half hours every other night is very different to recording in the studio. Live the stakes were much higher. In the studio he can take as many breaks as he wants, he can start over if there's a mistake, he can try tricky part over and over, and so on. Those are luxuries that he obviously can't afford on tour. Also, live he had to play thing that he recorded decades earlier, and he wanted to get as close to them as possible. That won't be a factor in the studio, where the drum parts can be written exactly to the playing level he's comfortable with at the moment.
If they want to record with Nicko, there are ways to do that and get pretty good performances out of him still. The question, imo, is: Will Nicko record anything at all? Only a few tracks? Everything?
Given how they're bonding with Simon it would surprise me quite a bit if Simon wouldn't appear at all on any future studio work.