Okay, well, donationware works, per se.
Kovid is also got a few other things going on than Calibre
That PhD from Caltech probably comes in handy.
And yeah, I do work on opensource software, too.. and I make money off the donations.. but we're talking beer and pizza (okay, and some really fun gadgets every once in awhile) money for the most part. Not mortgage and car payment money
I think many authors would find it hard to survive on the generosity of others.