Good question.
I had a very long discussion about this with a coworker of mine who has been on minoxidil 10+ years. He looks great, by the way. He says that indeed, after 2-3 years the regrowth stopped... but, he maintained what he had and continues to keep it.
Think about it, considering the hair life cycle, keeping the hair I have plus 2-3 years of regrowth is not a bad proposition if you can manage to keep minoxidil in your daily routine? He swears by the stuff, now has a full head of hair albeit a high forehead but says that 10 years ago he had the SAME hairline, and it was thinnning rapidly into the ol' horeshoe pattern! Seeing his relatively full head of hair at age 47, no vertex holes at all, inspired me to go for the minoxidil. Just my opinion.