The point is, you wouldn’t be developing for a Purism-specific phone or OS. You’d target GTK4 (or GTK3+libhandy), so apps will work on Gnome, Ubuntu, Fedora, desktops, laptops, the Librem 5 and any other Linux phone using that stack. That includes the most popular OSes on the Pinephone; they all use the libre libraries, compositor (phoc) and shell (phosh) originally developed by Purism.
(I don’t know how well-behaved GTK apps would be on KDE Plasma Mobile.)
Also, don’t base you decision solely on issues and anxieties people aired in 2019, four years ago and a year before the Librem 5 started shipping. That would be akin to ditching SailfishOS based on the opinions of some disappointed (and very vocal) backers of the Jolla tablet crowdfunding campaign.
I mean, Purism rightfully deserve criticism for a number of things, but they also deserve recognition for what they have actually done for the Linux phone ecosystem.