Piper managed to squeeze out over 4000 PA31, most of which did not immediately start life as commuter workhorses.
That's true, but maybe it's my perverse interest in maths that I want to look at it a little closer -
PA-31's on the register according to CASA - 183.
I'm sure there are a few good, low time ones out there (I certainly haven't seen them, but statistically at least, they
must be there!!) and I have flown roughly 7.7% of these (14). Only one of them have been below 10,000hrs. 2 years ago 2 of them were approaching 20000. That is by no means a huge statistical sampling, but it is a little telling.
I think the thing that will ultimately ground them is that they will just get too expensive to repair/re-engine.
Oh, and 35 404's on the register and 33 Caravans.
Using your 4000 (it's a nice round number!) PA-31's built, roughly 4.5% of the total ever built have found thier way downunder.
If past trends predict future trends, the small number of possible replacement aircraft that may or may not be in the pipline (like the Tecnam 2012? But specifically that one) that will actually be built, will probably not be built on the same scale as they were in '65-'85. So we might reasonably expect to see 10-20 of these mythical creatures over a twenty year span to replace roughly 250-odd airframes!
I don't know how this will all pan out in the future, but as someone pointed out earlier there will be a pretty big hole in the well trodden ladder of pilot progession in the coming years.