I’m not sure whether the community will take over or not if Purism disappears. libhandy has already been widely adopted by GTK/GNOME developers, and 58 people have contributed to its code so far, so it will certainly survive. As for the rest of the Phosh mobile environment, I’m not sure after looked at the number of commits to Plasma Mobile, UBports, LuneOS, Maemo Leste and Nemo Mobile in the last year. The only community mobile project that is really working with 100% volunteer labor is Plasma Mobile (and it does have a few paid developers from Blue Systems). Lomiri only had 75 commits in the last year, and still hasn’t completed an upgrade to a newer version of Qt since Canonical abandoned it in May 2017. LuneOS doesn’t appear to be doing much new dev work, and is mostly repackaging the code being released by LG. I doubt that Nemo Mobile’s new Glacier interface will ever be usable for normal users. UBports only has 9 devs registered in its github account and Maemo Leste only has 4 devs working on its code.
The Mobian and postmarketOS devs do contribute to Phosh and I count about a dozen volunteers who have contributed to the Phosh mobile environment (apart from libhandy), but I doubt that they represent more than 3% of the total commits. At this point, Phosh is doing better than the other community mobile projects except Plasma Mobile at attracting outside developers and packagers, so I predict that some people will try to continue Phosh as a 100% volunteer project if Purism dissappears, but I also think it likely that the Mobian and postmarketOS devs who currently contribute would switch their efforts to Plasma Mobile, especially now that Plasma Mobile is adding support for GTK apps and getting rid of libhybris.
It is pretty easy to attract volunteers who will do testing, file bug reports, package in distros, and even develop an app or two, but I don’t think that it will be easy to attract quality volunteers to do new dev work on phoc, phosh, feedbackd, etc. if Purism disappears.
With a year or two more of Purism working on Phosh, it becomes more likely that the larger GNOME community will step forward to take over development if Purism disappears, because at that point, there will probably be over 100 GTK/GNOME applications that have been made adaptive to run on mobile devices, and many of the developers of those applications will start to feel invested in the success of Phosh and start to contribute. Plasma Mobile works because the larger Qt/KDE community now wants their applications to be able to run on mobile devices, and we need the same commitment to happen in the larger GTK/GNOME community.