Is the durability of modern-day Kurzweil/Fatar actions better than their counterparts from 15 years ago?
Yes.
I demolished about six or seven actions in my PC88 in just a few years. It took me about four years to beat one PC2X action into Fatar Flakes. I'm not sure which Fatar model actions these were.
The TP40L in my PC3X is a different animal. I just started using it on gigs a few weeks ago. After twelve sets in three days, I experienced something I haven't felt in years. A little fatigue. A little sore in the hands and forearms. I'm obviously using some muscle I haven't needed for a while. This probably means it'll get more of a beating than the PC2X.
Based on what Dave has told me about the testing of the TP40L, it should take me a long time to find out for myself exactly how much the reliability has improved. This is something they took very seriously, because some customers took a hissy fit on them about the old stuff.