I think it's fairly safe to say that uploading to youtube at higher quality is beneficial. Your whole thing about how Youtube 480p quality can be matched by cheap ancient SDTVs if anything just provides extra reason to do better, especially seeing as many people are going to want to watch the video across their screen while not doing anything else. Lower-than-SDTV quality blasted across a 23" monitor viewed from 3 feet away is not ideal at all.

: If you say so.

It's similar to why audio is sampled at 44.1kHz even people typically can't hear frequencies above ~20kHz. Sampling at the frequency (or resolution) of your final output leaves you with lots of artifacts; in the case of one-dimensional signals like audio, anything with a frequency of greater than half your sampling frequency will be inaccurately represented. Sampling at 20kHz would only accurately capture sound components below 10kHz.

Supersampling images by rendering them at a high resolution and scaling the result down is a very expensive approach to antialiasing, but it deals with every kind of spatial aliasing ever, and even improves your texture filtering.