I don't think prog has gotten stale, though 2015 was a fairly weak year for the genre, to be sure. I personally had 1 full length that definitely counts, 1 EP and then Symphony X which probably counts, though that particular album could also be seen as prog/power as there's a ton of power metal on it. In 2014 I had 4 in my top 30 and a whole bunch just outside of it, and this year already I have a 5 star album (The Astonishing, lol!), 2 4.5's and the Headspace album is just outside of a 4.5 and could get there with more listens, so it's already looking like a better year for me. There's also new Haken on the horizon, and their last album was part of a very strong 2013, so we'll see how things go the rest of the year with prog.
One thing I will say, though, is that prog has become like symphonic metal, in that more and more bands are finding ways to mix elements of it into different genres, so in general there probably have been less standout pure prog albums, just like there have been less standout pure symphonic albums, just because of the merging of sounds.
