If the batteries degraded 15% per year then Roadster owners would be totally screwed by now. You'd be talking max charges of only double digit rated miles by now for some if that were the case.
But this is the opposite of reality, fortunately, and the batteries do not degrade substantially.
I personally just lost my first rated mile on a range charge, though... at just over 15k miles. So lets see... thats a 0.38% loss in range after 15,000 miles.
If I extrapolate that out to 250,000 miles that comes to a loss of about 6% range, or about 17 rated miles, at that time... I'm quite fine with that. And assuming I still have my Model S by then I'll still be just as happy with it I'm sure.