What if your intent is to shift to a time when Youtube doesn't exist?

It when you don't have access to it (plane flight, etc). That sounds like time shifting to me.

If, for example, you bought a movie and downloaded it to watch on a plane when you can't have access to streaming, why wouldn't it be legal?

That would be an interesting argument. I suspect it would be illegal because a pay per view stream is typically time limited. But if the stream itself were still valid while you were on the plane ... ?

What about using youtube-dl as the only technical way to play it on a low power device, like with mpv on an Rpi or old laptop.

That's a kind of tech adaption of the streaming/playback mode (and I've done it a ton on previous laptops)

Then RIAA would argue for a DRMd version of downloader that is prevented from doing it in high power deviced.