I don't see the issue of using AOSP. You get to skip the many years that Sailfish OS will still need in user testing. You get to skip all the possible incompatibilities with Android apps through the compatibility layer. AOSP is also Linux on the phone. I guess you mean GNU/Linux on the phone, but AOSP now also has official support for a Linux VM (you want a VM because traditional desktop Linux security is not great). They are even adding support for running Wayland apps. With the recently-added desktop support, you can plug a phone into an external screen and you'll have a desktop with Android apps and Linux desktop apps.

I think the chance of Google completely closing AOSP is pretty small, AOSP being open maintains a power equilibrium between Google and other OEMs. Closing up AOSP carries the huge risk that Samsung and some other big OEMs will fork it and Google has essentially lost the whole market overnight. I am pretty sure this is why Samsung phones also have the Galaxy Store with a bunch of apps like Netflix in it. The Galaxy Store is Samsung's subtle message to Google saying: don't try to rein us in, we can cut you out.

That said, even if Google closes AOSP, forking it and maintaining it as an open project is going to be far less work than brining Sailfish OS to the level of polish, security, etc. of AOSP.