I obviously started out talking about high refresh 4K monitors because that's what you first wanted to know about. My intent there was to let you know there are very few of them, and they are very expensive.
??? I was referring to your 2nd paragraph in the OP, before I'd said ANYTHING, so what's this reason you said that bit?
I'll clarify what I mean, you were seeming to feel you needed a 4K monitor to cover all the res options you wanted to play at. I was mostly referring to playing at high refresh at 1440p on it, but truth is, as I've shown you with TPU's performance summary, there's a LOT of games that can't manage even 45 FPS on a 4K display. So maintaining decent FPS in general is hard on a 4K display, high or otherwise.
DSR (and AMD's VSR), allow the game to render at 4K, then a filter is applied to sample it back down to your screen's native resolution. The result is way more clarity with same amount of pixels. Truth be told you really don't NEED extra pixels unless you have a very large screen. This YouTube vid both explains the tech and shows the result.
https://www.youtube.com/watch?v=_0v-wcOijsg
I'm willing to bet you might have at least SOME older games your 1070 could handle at DSR 4K, unless your CPU is very weak.
The reason resolutions below a display's native don't look as good is because it involves scaling. You can either have the GPU do the scaling, or the display, but either way, it will not look as good as native res without scaling. I know this by experience because I at one time had a Sony X900E 4K HDTV, which has industry leading scaling and upconverting, and I still noticed playing at 1440p was not as sharp. Even in Just Cause 3, a series known for lots of blurred distant textures, it was noticeable.
Regarding a 1440p display vs 4K, I was saying given the proper GPU and CPU power, games you can play at 4k would look just as good on a 1440p display using 4K DSR. DSR in itself makes 4K displays kinda pointless except for the rich, especially if you're talking high refresh rate ones. DSR 4K wouldn't look better on a 1080p display than on a 1440p display though. Either way you're simulating the same 4K res and it takes the same GPU power, and you'd have more pixels with 1440p The only other factor would be screen size, meaning it would look a bit sharper on a smaller screen due to smaller pixels. This depends on viewing distance though. The only reason I mentioned using it on my 1080p display is to convey how well DSR works in general, but if the screen size and viewing distance were equal, it would look even better on a 1440p.
TPU is VERY thorough about using same exact test setup for each game, and explaining what settings they use. You have to go to the test setup page for that, where they explain every game is set to the highest settings, unless mentioned otherwise on it's performance page. Pay attention to hardware spec they use though. If you can't run any of your games on DSR, you're either setting up DSR wrong, or your spec other than GPU is very weak.