What if I decide to stop paying maintenance on my perpetual license?

Please don't.  For starters, you can't get support from xTuple or any certified partner if you're not current on maintenance.  Moreover, it's far more expensive to get back on, after you've gone off maintenance.  Not only do you have to make up all the back payments, there's an additional penalty as well, which starts at 50% of the amount owed.  So for example, if you were two years off maintenance on 10 users, and maintenance were priced at 10% of $3600, or $360, then you'd be looking at the following:

Back Maintenance owed: $7,200
Penalty: $3,600
Total Due to get right: $10,800

Was it worth it?  Just to avoid paying the $3,600 a year, or even $360/month?

And that's assuming your two-year old data was in a pristine state, and didn't need any consulting services to go in and clean up - which has never happened in the history of software.  So in short, don't do it.  If you're worried about cashflow, take advantage of our monthly billing option.  But there's rarely a happy ending to a broken maintenance agreement.