Downscaling is also a substitute for frame interpolation. 24-60 fps video becomes noticeably smoother, but the effect is weaker with animation ≤8 fps. Why does this even happen? That less of an absolute difference between frames is less jarring makes intuitive sense. However, I can't find any research, only the seemingly single forum thread where it's been discussed on the internet.*
Finally, I contend that even with a high-resolution source, a smaller viewport is not necessarily a worse experience, merely a different one. Because peripheral vision is so poor, by looking at a smaller image, you see the 'bigger picture' faster, without having to move your eyes. It's most noticeable in fast montages where you'd rewind to see something that you missed.
See also the opposite: "attention deconcentration"
PS: Downscaling below 1:1 increases sharpness with little detail loss when the original resolution is higher than the actual level of detail, owing to the likes of intermediate upscaling, optical limits, or compression.
I agree, most of the time it's fine. I agree, people should get the choice of the original vs the upscaled.