All this chat about upscales is most amusing. Most 4k blu Ray films are upscales.
Anything shot of celluloid on 4k blu Ray is a downscale as celluloid has a much higher native resolution than 4k
All decent 4k TV upscale.
You can go on forever. What matters is the quality of actual film or TV material. Upscale downscale, so what as long as it looks good.
Virtually everything we watch is upscaled or down scaled.
It's a red herring
True, but, IMO, not exactly a red herring because:
The closer we get to (or exceed, even) our own ability to "see", the less important differences in detail become. That's affected, of course by any number of factors including, but not limited to, display equipment, closeness to the screen vs. its size, and of course our own visual acuity.
For many, I suspect, at normal/comfortable viewing distance (vs. screen size), regular HD vs. UHD is so close to the limit of perception that differences are either wholly or substantially invisible - even if we compare exact 1:1 pixel mapping where source and display match vs. something that mismatches and is scaled one way or the other (as long as it's done well, and preferably just once).
However, I'd suggest that SDTV is probably (in this context) so much further short of this limit of perception that we are more likely to be aware of its inferior detail.