Earlier quoted context omitted.
Another reason to backup HD Youtube videos is that they'll "demote" them to SD if they don't get enough views.
Is that true? Do you have a source on that?
Maybe I'm an isolated case, but many of my videos from 5 years ago with few views.
They were filmed on a Nikon DSLR in 1080p. A few weeks after uploading they were fine, 1080p. Several months later, I noticed a few were 720p, and now many are 480p, but a extremely poor quality 480p that looks more like Maybe it was the codec Nikon was using, but regardless, they're basically gone, and I no longer consider Youtube a "safe" place for videos.
I would send links, but I don't want to dox myself.