The camera shutter is a mechanical part, and like all things in the universe, subject to eventual failure. You can see How many actuations are "too many actuations"? for some information on just how many clicks a typical shutter mechanism is good for (depending on the model, between 50,000 and 400,000). But note that these are just estimates based ...
Either a stuck mirror or a stuck shutter.
Remove the lens and use the camera menus to navigate to the setup for manual sensor cleanup, which should lift up the mirror fully and open the shutter fully. You'll quickly sees which one is stuck (unless that maneuver fixes something...).
In any case the camera likely requires service.
It feels awkward for spending so much R&D for a sensor technology and then abandon it.
Do you see it awkward to spend much R&D on film and then abandon it?
You know, Kodak built the first ever digital camera and then abandoned it. That was very awkward, far more awkward than what Nikon did, as we all know the digital camera later reappeared and ...
A count of just 15 seems unlikely on a used camera!
No, it won't turn off at some predetermined number. It's just, like an odometer on a car, an indicator of how much it's been used. Interesting when determining the resale price perhaps.
But no, shutter replacement if it DOES fail is not a DIY job, and is very likely not economic.