I agree that different owner's experiences with airframe parts availability can vary greatly.
I was not involved in either of the two mentioned parts availability problems as the owner, but as the DAR (like FAA DER) asked to provide approval for a "made part", when Piper would not support the aircraft with the required parts. The quote I reference was spoken by Piper to me.
The Seneca II I was involved with was unairworthy as a result of corrosion - just certain parts, but critical ones! If there were any alternative sources for the required parts, a great effort expended, before I got the call, would probably have revealed it/them.
Similar challenges with Cherokees, and in particular an Arrow I was asked to inspect, have resulted in a similar response from Piper.
Yes, Cessna has mandated comprehensive inspections, which, in the era of "aging aircraft" for larger types, does have some merit. I believe that Cessna will still provide the required parts. (though I have not personally checked this). I attended a Cessna presentation on this subject, and nothing in that presentation lead me to think that Cessna did not want it's legacy twins flying, they were just intent on assuring their airworthiness. I understand it's beginning to filter down to the singles soon too.
I did check with Piper myself, and was twice told "No". That makes an impression upon me....