As I understand it, while the core pieces required for Linux to run on Apple Silicon will be upstreamed, there are parts that smooth the experience out and make it more practical that are unlikely to be integrated into other distributions, which necessitates Asahi's continued existence as a distribution.
It should be noted, Marcan has said that the ultimate, eventual goal is for Asahi to no longer have to exist as a distro. That may be a long way off however!
This is a similar model to the KDE Neon distribution, which upstreams basically everything but gives some benefit to developers working on the KDE project.
