New games get more demanding, new GPU generations are on the horizon. That's how it works.
Kinda. The 290 came out in 2013, and the 390 is basically that 2013 card with bells on. Relatively to that 2013 GPU, the 2014 970 is falling behind in newer games.
The problem isn't that new GPUs are beating the 970. We don't have newer GPUs than the 970. The issue is that the older 290 is starting to beat the 970 in newer games even at 1080p.
If you are a zero day gamer the 970 is obviously the better choice because Nvidia will have "Game Ready" drivers for you. But if you are a "I will buy it when it goes on sale" gamer (and I am one) often by the time you buy it the older AMD GPUs have caught up or are ahead. The Witcher 3 was like that, AMD GPUs started out way behind but then caught up. And that isn't even accounting for the fact that those 2013 GPUs might get an even further boost under Directx 12. Two years from now the 290 might be the clear winner between it and the 970 thanks to the new API.
You might say "well most people will have bought a new GPU by then," but I don't believe it. I bet a good chunk of people who just upgraded to a 970 since 2014 (making it the top GPU in Steam) are going to sit on it for at least three years. What we witnessed last generation is the sweet spot for GPU purchases moved from the $200 price point to the $300 price point (I think because the 960 sucked so much compared to a 970). With all those midrange gamers now paying 50% more for the GPU you can assume they will want to keep it longer than they kept the 660/670 they replaced. If the 970 really does require a lot of driver tricks to keep the 3.5GB thing from being a problem, then within the effective lifetime of the card for most 970 owners they might watch as their GPU suffers relatively due to a lack of focus from Nvidia. That isn't a good thing.
AMD was simply the better decision to make in the midrange last generation. Especially when those 290Xes were going for 970 prices. I really kick myself for missing that boat. An aftermarket 290X might end up where a 980 currently is in the pecking order in a few years.