It's not a big market for aeroplane engines folks.
The costs to develop, vs the return, are not worth the investment. Coupled with the fact that right now, the engines produced can be installed on your 1940s Cessna to your brand new one, and you see why the engine manufacturers are resistant to change.
For what the engine is designed to do, it does it acceptably and reliably and therefore does not require more modern variants.
Wally how old is your beloved PT6? It does the job reliably, it could be made far more efficient (like a Garrett
), but there are many installations on many different airframes doing the job well, therefore there is no reason to change the design!
If you start putting overhead cams with variable timing, ECUs, higher engine rotation, etc etc, you are just creating something that has more to go wrong!
Aeroplane engines need to be simple, reliable and as redundant as possible. Modern engines do not fulfill this criteria.