IMO the problem is that the devs who hoped for donations have already found their way. Flattr does not add that much to PayPal and similar. It is too little too late.

OTOH, even if we were generous and assumed that as many as 10% of all Jolla users would actually buy a paid app, and even being extremely generous and assuming it were the same app, at the current number of Jolla users worldwide that is still what? About 2000 buyers? Even if charging $50 for the app, that makes $100k. That barely covers one developer's year's salary plus running costs. And remember, that is with some very generous assumptions.

So it may be just as well that Jolla did not bother with a proper paid app support. Half measures like Flatter may be good enough.