On one hand he talks about charging for his plugins, and on the other he says that open source is not sustainable unless somebody pays for it.
Yes. Somebody does pay for it. Either the programmer himself, or the company that employs the programmer. But somewhere along the line somebody, somewhere is paying for open source programming.
Yes there's a bit of quid pro quo. I'll use the gcc that somebody sweated blood, tears and money over for free. But the gcc that I use today allows me to put my software out there 'on the cloud' for the next person to use for free. Have I sweated blood, tears and money to produce my software? Yes, of course I have. Do I want cold, hard cash for that software? No.
There is a old principle here: 'Pass it around' or maybe 'Pay it forward' is how you first saw it. But I received gcc for free from upstream, I pass my own software downstream for free too.
"And so it goes."