I have been in similar situations back in the day (ie, $400k paid off former primary that generated $1000/mo in net income) and have always chosen to sell.
My thinking is this: no, I don't *need* to squeeze out more return. If I only need $36k a year to live on, hypothetically, and I have 3 of these $400k properties, then I'm golden! No need to chase more returns via a better performing (ie stocks, a different rental property, etc) investments.
The problem is that having a big chunk of income from just one property is that you've got very concentrated risk, it's a bit like having all your money in one stock. A natural disaster, big change in society/employment prospects in your area, new tenant-friendly rental restrictions coupled with inflation, or just a frivolous (or non) lawsuit could leave you high and dry. They're all low probability events, sure, but you're totally screwed if they happen.
If you have plenty of money in other investments, it's not a big problem. If you're going to make this one property your entire nest egg, terrible idea. Your strategy for what to do should be partially based on the return you expect, and partially based on your life plans/financial situation/risk tolerance, like anything.
-W