This is a trend happening across all classic themes. Pirates 2009, while being fairly generic, was a very good theme. Pirates 2015 was a clear recession. The same can be said for Kingdoms and Castle 2013. Space is next up, I expect a new Space theme soon that introduces nothing new and is discontinued after one year.
I very much agree with this, however, in other themes what I've noticed is that you have a bad year for the theme but then a good one next time the same theme or a remake of a set comes out.