Been waiting for years now

If you're on android, try searching on google "OGYoutube"

Use soundcloud for new free music anytime!

It does its called music key

just play the video in a web browser then turn off the phone, go to control center on the lock screen, then press play.
Edit: On iOS

They do this so that the ads will play

Fuck sakes, this again. It's a video app. There's no logical reason for it to play if you can't see the video.

Seriously, stop posting this. It's common sense.

I purchased the YouTube music key which let's me download music and play it without Wi Fi and with YouTube closed

The website can.

*go cows*