I assume some people decide to use
+++ YouTube Music +++ as a streaming music source. If so, then their music source is videos.
Not necessarily.
"With a YouTube Music Premium membership, you can choose whether to listen to your music or enjoy the music video experience. In song mode, you can listen to the music without loading a video. In video mode, the music video will play whenever one is available."
https://support.google.com/youtubemusic/answer/6313574I guess they feel a myopic user's money is as good as a visionary's.