Makes sense. Multiple PCs certainly gives you a lot of fun upgrades down the line as you can pass off the CPUs/GPUs from your primary to secondary to tertiary systems, etc. This justifies upgrades
As far as the performance leap from 7900GTX/X1950XTX, yes for sure it's justified but as far as longevity it was also considered to be one of the most legendary cards but I think I'd have to split 9700Pro/8800GTX as the best cards to come out from a performance jump point of view but longevity factor I'd give to 7970 over those ones.
We'll see what happens though as GPUs continue to increase in performance but with static PS4/XB1, games aren't getting as advanced as fast as I thought. At this pace the 980Ti might last a while but we'll see.
The thing is it was possible to buy an after-market 290 for $200-250 well before 390 dropped. Even today it's not easy to find a 390 below $280. Think about all the gaming enjoyment someone already got out of a 290 for 1.5-2 years? I doubt the 390 will outlive 290/290X due to 8GB of VRAM. Also, I think comparing 390 to 970 makes the 390 look pretty bad since the 970 came out Sept 2014 and is very similar in performance but cost $330. Imagine if someone waited 1 full year thinking there would be something much better than an after-market 290/290X/970 as of October 2014? Well that never happened in 2015.
8800GTX caught a bit of a break because Tesla wasn't a massive enough jump to make it redundant, and Fermi was so delayed and took awhile to get its 40nm issues sorted out with the 580. It took three years until the release of the 5870 for a card to be twice as fast as the 8800GTX.
For the 7970, the 980 wasn't quite twice as fast as it so it took a bit over three years until the Titan X was twice as fast as the 7970. The delay and cancellation of 20nm cards really helped keep the 7970 competitive with newer generations there.
I think a lot of the reason why the 7970 feels like it's aged so well is that it still does really well in most games at 1080P. You might have to turn down some settings here or there in some, but it's still good. For the 8800GTX, it wasn't too long before games like Crysis and FC2 came out where you really had to dial back settings and resolution to make it playable. Even if it took awhile for the cards to improve drastically because G80 was so good, it still seemed like it was limited just because the games punished the hardware so much at common resolutions.