After a decade or two of maintaining a complex, "grown" application it might be become fun again. You really know all the quirks, all important patterns and call chains. You know the pains of the customers/users and, if you have the time, might be able to do something about it.

Frankly, starting every other year on a completely new application/system does not sound so much fun to me, now that I am programming for more than four decades.