[verin0x]

In theory yes. Asteroidos has experimental support for a mainline watch. Most vendors don't upstream their drivers and kernel mods. Also Android drivers use an abstraction layer and a different format to some extentent. So you have to reverse and write your own driver.

So they could run mainline if the vendor or a user bothers to upstream drivers and hardware quirks.

A lot of the vendors don't meet quality expectations of the kernel team and sources are usually for older kernel versions and the code would need changes or refactoring.

[zozbot234]

The mainlining work is usually done by the community, not necessarily vendors. The relevant pmOS wiki page is https://wiki.postmarketos.org/wiki/Category:Watch but it may be less comprehensive or up-to-date than the AsteroidOS hardware support list. Everyone can help by adding technical info about hardware to the wiki!