Ultimately, I doubt that every piece of media that is currently on YouTube will be available for as long as I need them. That could be a year, or it could be fifty.
For my purposes, if a video gets taken down, it's lost forever, I probably won't be able to find it again.
But 'Guinea Pig Noises Loud Squeaking Sounds.mp4' lives on my hard-drive forever. ;)
I feel the same way about webpages really. wget -> it's mine until my hard-drive and all backups die. bookmark -> it's mine until my hard-drive and all backups die, PLUS the site goes down. Also immutable, which can be a bonus.