No it is not only that. It is a quicker road to save millions of older devices from being senselessly recycled.
Yes, fsck blobs, everybody hates em. But meh whatever, giving many more developers than only nexus device owners access to a linux phone stack is the best way to start improving the UI components (Plasma Mobile, Sailfish, Liri OS, coming multitude of wayland compositors .. .), while the clean method pursued by PostmarketOS would be perfect as an upgrade option to Halium once critical drivers have been implemented for each device.

Yeah sure. But whatever. Some of the problems can be shielded. And why leave devices just with android instead of creating a universal linux distro (alpine looks very interesting, or debian, or a mixture) that can be installed on top of any android that can be rooted.