More-or-less agreed.

> This heuristic isn't fully fleshed yet

I'd perhaps extend it as:

“Don't work on FOSS you or people you sufficiently care about aren't using anymore.”

Who qualifies as those you care about and why is an entirely personal thing and I'll not judge anyone's choice there. They could be friends, they could represent a cause you support, they could be randoms and you just get off on feeling helpful, they could be important and you like to be associated, you could be being paid to care, etc.

Also it goes without saying, but I'll say it anyway as I'm in a wordy mood, if you are enjoying working on something even if you don't use the output I'd still consider that to be finding some use in what-ever it is.