I still have an argument for "time and materials" - original article is about opportunity cost. You can continue working and being paid for working on X - all

I still have an argument for "time and materials" - original article is about opportunity cost. You can continue working and being paid for working on X - all is fine. But what if in that time you could get customer that is paying more - will you still be willing to put up with demands to continue to meet "higher standards"? Will person demanding "higher standards" pay more - so would you renegotiate?