> Why not drop to a "did you want the playlist, or just this video?" prompt?
Introducing breaking changes like this are no longer an option once your program has more than 5 users.
In addition, well-behaved command line programs aren't supposed to gratuitously prompt for input anyway.
Side-benefit: force care to be taken to be explicit if used in a script.
Personally, without reading any docs the first thing I did was paste in just a video ID (i.e. not the entire URL) so it seems natural to me that including both would be ambiguous.