He cut his salary by $70k for 4 years ($280k 'lost'), and now he's earning $100k which is $10k more than the $90k he was on. It'll take 28 years to make up the lost salary.

I mean, the math is very much simplified. People get raises, and change roles, and there's tax to deal with, and jobs have benefits, etc. The core point is that you'll probably earn a lot more in a job. If you can find one where you're paid to maintain a library you like that's probably better for you financially than trying to get sponsors.