I'm sure this will be possible in future Vision Pro devices, main issue right now is probably performance. Macbook Pros already can only push to 2-3 monitors (albeit at higher resolution) and the Vision Pro is also running its own apps simultaneously alongside the Mac window (curious if it is the Mac or Vision Pro doing compression and other processing work for streaming to the headset). Apple being obsessive about frame rates and performance are likely erring on the side of caution before allowing too much. For example the recent downgrade from 1080p to 720p when it comes to streaming the Vision Pro screen to other devices.