One thing to consider is the quality of the gas piston. You get a bad wound wire spring, it breaks and for about $20 you can get a very well made replacement. If and when your gas piston goes south, you have to replace the entire piston. Not a problem today but, twenty years from now, I bet I could still find a spring for a Diana, etc.
Back in the days before internet on of the leading airgun mag writers used to hunt with a theoben sirocco always saying how brilliant it was and when I get back into air rifles this year there are gas rams being launched at more affordable prices. Back then I got laughed at for having a pneumatic pump. They weren't real guns. All the Target boys used hw77s and the like even though my gun shot without a wobble. now pcps are the dogs and most shooters I come across use them but the number of springers made and on offer is vastly larger than in the 80s with technological advances at 120 gbp 250 gbp and 350gbp plus. So the market will change but springers are still going to feature strongly for a while longer. Their simplicity tunability and the fact that just need cocking to fire. A screwdriver and basic tools to maintain and tune. Will always be attractive. Why do people run old cars. They like to maintain them. Why do people spend a fortune on modifying cars when the same money could buy the next model up that is already quicker. Because its satisfying to think I tuned that up. Springers for ever.
Until the quality and consistency of the NP is improved, I don't think there's any risk of the spring going away in the near future. The tuning possibilities up/down is also a plus.
what issues have the nitro piston had
It has crossed my mind that the old springers, as we know it, may be on the way out and gas pistons becoming the new standard.
I recently got an HW90 Theoben. At 26 Bar pre-charge (377 psi) it's a bloody violent beast. When that gas strut unloads you know it and so does whatever I'm pointing it at. It destroyed a Bushnell Trophy XLT in fewer than 200 shots.I'm going to mount the replacement on a dampa mount and cross my fingers. I wonder how many times Bushnell will replace the scope before they start asking questions.