I can see 4k with DLSS. I can't see 4k 120fps. A lot of the AAA games on the switch right now are running lower than PC low graphical settings... So while the switch can play them, unless it's the only gaming device a person has or wants to play their games on the go, playing it on other platforms is a much better choice. We will see how next gen goes if XSS is a thing and how Nvidia's hardware improvements come into play.
I think third parties will use DLSS to try and allow the graphics to be closer to the next gen consoles rather than giving the player a high fps or even high resolution. My biggest concern would be the cpu since zen cores are no joke.