Bringing up new hardware to run with a mainline kernel takes a lot of time, so the Jolla Phone will initially be shipped with an Android kernel + libhybris-based build.
However, this does not mean that the phone is doomed to run the same software forever. Jolla intends to make it possible for users to change the operating system.
I think it’s quite likely that the community will be able to get postmarketOS and a mainline kernel running on the phone sooner or later. Since the new Jolla Phone seems to be more popular than the C2, I’m hoping that more people will contribute to such efforts.