Superhero themed pins do not interest me at all, why would stern release another superhero pin just after x men  

Because they are cool, span generations, are tried and tested,will still be around after ALL other themes are distant memories and they have way more appeal than generic band/bland pins.
If the kids of tomorrow have any interest in pinball, which I doubt very much they will ( we are the last generation of pinheads imo) then the first games they will be drawn to are the superhero pins based on films, tv shows, comic books etc that they grew up with.

Having said that, I would love to see JJP or whoever take one or more superhero themes to give buyers more options than just Stern who are the only one on the ball on these themes atm.
I would love to see a Flash or Aquaman pin based on the comic books.