Hmm, I always bought "outdated" videocards. They tend to be cheaper when they're getting eol status.
But tbh, ran my 7970Ghz for 2 years on my pc, and it's still working today in another pc, now the 390 and looking at the games I play, it will do for another year or so. And probably more.
As for 4k, can't run that on my 1080p monitor, but 1440p vsr does look nice, and plays good too, depending on the game. Fallout 4 looks and plays good at 1440p but some other titles not, like the ingame fonts becoming too small or simply too demanding etc.
So for now and the rest of this year I suspect 1080p will still be the standard, especially considering the console ports we get more and more.
So outdated? Yes, most definitely. Something to worry about? Certainly not. It will perform as it should, if it breaks down you can rma it and probably get a new working card.
As for DX12, still runs the same or worse on most games compared to dx11, nothing to cheer about on that front, unfortunately.
Vulcan is absolutely fabulous in Doom, gave me about 30% increase in frame rates. So lets hope more and more games will use this so your and mine "outdated" card can still maintain +100 fps @ultra settings, which is actually outstanding compared to games and hardware 10 years ago.
