One of the things DF (John) pointed out, I believe, was that if Halo Infinite is gimped by having to be compatible with Xbox One, that will affect Halo Infinite for its whole lifecycle. Remember we're not just talking about a normal game here, or even a normal "tentpole" game, but a platform for GaaS -- a foundation for a 10-year plan, similar to Destiny. If you start with a gimped game as a foundation, you're stuck with that gimped foundation for 10 years.
I can understand why MS did it. They don't want to leave that 40+ mil. install base behind. They are prioritizing GP subscribers, who love 'free' access to the Day 1 AAA MS exclusives, which have been pretty scarce lately. I don't think MS are going to change their mind about keeping it compatible with X1.
I also wonder how much Gamepass itself is holding Halo Infinite back. It seems to me that if your ambition is to have your games stream not just on the XSX and PC, but also on cell phones and the like, then you've got to make it compatible for those devices, too, at some level -- not to run natively on them, of course, but at least to stream to and display on them.