They kept it multiplat this gen because they acquired the IP mid gen, it was nice of them to keep it multiplat since some people had already made their platform choice at that point and they didn't want to have to force people to buy a 2nd console in order to play Minecraft. But next gen is a fresh start, if they announce it's exclusive right from the start minecraft fans will know to choose Xbox 4 over PS5 for their first console of the gen. There is no reason for them to keep it multiplat when it can boost hardware sales for Xbox 4 massively and help them close some of the massive gap that we saw this gen.
I’m telling you, it would get huge amounts of bad PR and doing that while promoting cross-play at the same time would seem incredibly hypocritical.
I really don't think the backlash would be bad. It's MS IP, they can put it on whatever platform they want. If I was MS, I would force companies to let it be cross-play if they want the game on their system. If they don't play ball, neither does MS.