Taxpayers aren't prisoners, so there is some maximum rate you can get out of people before they start leaving. I doubt it's the case, but there is a non-zero chance that paying all their obligations are actually impossible.

It's interesting enforcing contracts when my local government can sign a contract that we'll pay pensions for the next 40 years in exchange for a service now (say education), but I can move away after consuming it (or simply never become productive and pay taxes).