Are you implying that "the devs lost interest" is mutually exclusive from "the devs were facing burnout from dealing with a hostile community"? To me, the latter is causal to the former, but you might disagree.
In a commercial setting, there is financial motivation for a company to hire and train new developers, but in the case of an open source project this may not be the case.
How do we motivate people to fix or continue development of open source projects? Is it possible at all?
Hm. Maybe it is impossible to motivate people to continue development of open-source projects. There are very few long-lived open-source projects, and very little open-source code. You're probably right that motivating people by anything other than cold, hard cash is impossible -- after all, what do people want in life besides money?