do some research on mello roos, by taking PUSD CFD #6 IAA as example.
seems mello roos is not only used for paying the issued bonds which has clear maximum amount, it also pays for some accounts which I consider as an open ended fund. you may think in 25 years, you can pay off the original bond, but actually every year, there are quite certain amount is allocated for other purpose than paying off original bond.
i.e CFD #6 IAA original intention is to pay off $13,000,000 bond issued in 2002.
a large portion of annual mello roos collected is not only used to pay for interests and principles of the 2002 bond. it also pays for Improvement Area Surplus Account, Lease Revenue Bond Account etc.
now I know why 25 years could be extended, since the payment is not really paid to original bond principles.