There are 2 reasons why Downscaling is bad
First it can crawl or not run at all on some pc if there cards don't support textures larger then 1024
which can happen on some sprites,also there is the problem of memory on the graphics card which is again limited
In that wallpaper example it could probably crash if my graphics card didn't support 2048 textures and also the example takes whooping 16mb vram
The second problem is the downscaling itself which you can lose allot of details and can look like crap
For example if one runs 1028 768 a 1080p image could lose allot of details and produce allot of artifacts as opposed to a 720 which is closer in range and as thus better defined