They will probably go back to selectively allowing BC. A company like Take Two would sure love to re-release GTA V, Red Dead 2, and Borderlands Handsome Collection (even Outerworlds since they also own that if it comes out this generation). Without BC getting in the way of their plans, If all 3 hardware publishers decided to say that is final, then the devs and publishers will have no choice but to submit. If either one came out alone making every BC just like that, it would be bad for them and their relations with 3rd party.
Even if everything won't be 100% backwards compatible, we can trust Microsoft to deliver a very strong lineup of BC games next gen as they have this gen. More so people won't have any trouble dropping their old Xbox in favor of the new Xbox since most of their favorite games will still be playable on the system along with all the new titles for years to come.
I feel like BC is going to be a very important thing for the first few year of the next generation due to the games as a service model being a big thing.
I'm really interested to see what games will be made BC or re released due to their GaaS model. What will happen with Sea of Thieves, Or Rainbow Six Siege? Will they be re-released, or made fully backwards compatible with graphical enhancements. R6 is on year 4 and shows no sign of slowing down, and Sea of Thieves supposedly has a "10 year life span".