Maybe he means it will be a PITA if the system isn't paid off by then, but I don't see how that would be any different/worse than selling a house with a second lien (e.g. home equity loan) on it.
We installed a 13.76kW system (43 panels) on our house last June. Our payoff period was pretty similar to yours, but I think ours was a conservative estimate. We switched to Green Mountain Energy which has the best net metering plan, and we haven't paid a penny in electricity since July. Of course, we're paying on the loan for the system which is a little higher than our previous average electric bill (ranged from $125-200), but we're in it for the long haul so that's okay.
I'd recommend going through your last 3-4 years of usage and figuring out what your average daily kWh usage is. That data can be used to size your system appropriately, and then you can track your solar production and see where you stand. In my case we need 46-52kWh/day to break even. As of yesterday my running average is right at 50kWh with the sunniest few months of the year still ahead, and the running daily average never dipped below 49kWh even with the colder weather we had this past winter.
If you have any other questions feel free to ask.