If one "prepays" tithing this year, and doesn't pay anything next year, then that person has become a non tithe payer.

The law of tithing is about paying 10% of your increase as you go, not "prepaying" one year for the next. Come tithing settlement at the end of next year, when your bishop hands you a form that says 0.00 paid, how can you honestly say in that moment that you are a full tithe payer. "Prepaying" just means you decided to pay more tithing the previous year, but says nothing about the current year.