I don't think the genre is stagnating, just the developers blindly stick to the usual formulas. Civilization, or Civ in space, it's the usual, colony rush, build, war (or whatever victory condition) end. Boring, done to death.
Polaris looks similar to that (Sowwy devs!) but I'll admit I'm not really paying much attention to it, and I've heard nothing but good things about Distant Worlds - even if it isn't to my tastes, however, my attention in terms of space 4x is going towards Stellaris (again, sorry!)
Why? No idea, just the Dev Diaries over there are interesting, well thought out, and seem to want to go beyond the usual cookie-cutter 4x build. There's some interesting ideas there, and the price point and the balance of mico and macroing seems to be a break from the conventional sprawling micromanagement-fests that 4x can turn out to be, - the alternative being 'small and tall' empire building, which isn't for everyone either.
I'm banking on Paradox to throw another curve ball, like they did with the "City sims are dead!" crowd by turfing out EA and their complacent, lazy SimCity series with Cities: Skylines.
Basically then, what I'm trying to say is, a genre only stagnates as far as the ambitions and creativity of the developer reaches. And as companies get successful and become bigger, both those factors get limited in some form, resulting in that until a plucky young upstart comes along. I'm sure that in the future, Paradox will go the same way, and maybe one of the Devs under Slitherine's wings will chunter along and become the new star! Though, in a perfect world I'd be happy with both Paradox and Slitherine doing well at the same time. ^_^
Just my thoughts, no offence devs!