If you already have a 780 you're probably good for a while. I'd imagine in another year or so you could upgrade to a mid-range card (usually Nvidia's X60 range) and stick with that for several years because of the console limitations.
It feels like these 900 series cards came on fast because they did basically bypass the 800's. I'm not sure why they skipped the 800 generation outside of mobile chips, but it's kind of weird. Maybe it is just to make people feel like they need to upgrade, heh. Or maybe it was so there was less direct comparison between mobile and desktop chipsets, but even that is odd.
But yeah, Nvidia will keep releasing new ones every year because that's the business they're in.