And seeing as the last one rolled out of Piper's doors around '84, I would wager about 75% currently in service would have been made well before that, and therefore probably won't see out the next 5-10 years?
A nice thought, however Piper managed to squeeze out over 4000 PA31, most of which did not immediately start life as commuter workhorses. Some even until recently have only accrued around 200 hours a year average putting them around 6-10 thousand hours.
If you add the other light twins, there was around 10,000 aircraft produced in the 8-12 seat piston market in the 20 years 1965 to 1985. This does not include the 6 seat and under class. The only aircraft that comes close to replacing these machines on cost is the Caravan and Cessna has only managed around 2000 sales since 1984.