I think the reality is that you get what you pay for, also what sells well tends to depreciate less in percentage terms.
You also seem to get few, if any issues of getting spare parts for a PA46.
With regards the argument about Turbine or diesel engines, it should be noted how may of the IO540 engined PA46's have had Jetprop conversions, its a logical upgrade on the PA46.
Bear in mind that a PA46 costs a fraction to operate of what a C421 did... Use for 300 hours per year and your talking of sensible savings on annual operating costs, even with the high capital outlay.
I think given time we will also see smaller versions of turbine proped engines.