downsampling is the same thing as running higher resolution. when I use 2560x1440 it looks better than 1080 and performs just like 2560x1440 does. and then 3200x1800 looks even better and cleans up the edges even more. going back to 1080 does look slightly more blurry and has more jaggies but its not a massive difference that blows me away.
It's really not the same, it's down to pixel density. Even though a game might be internally rendered higher, it still has to be displayed at the same pixel density. Higher resolutions with smaller pixels give a much sharper image, even I was surprised at how sharp it was when I saw it for myself.
All downsampling does is give a higher quality AA which is great, but not the same.