And isn't Kanye also still popular by those metrics?

However, measuring said relevancy is a very subjective endeavor (due to the fact that it’s hard to objectively evaluate things outside of our own bubble). In my view it’s whether the artist is “talked about” and whether they are influencing the current music (and a broader culture) - that’s what makes them relevant. Kanye is still doing both of those things, so he is very relevant.

Donda (and Donda 2 to a smaller degree) was a very noticeable cultural event. Listening parties, album guests, the controversy around his romantic relationships (scandalous IG posts, etc), merchandise (tied to his big fashion releases)…

Billy Joel has none of that. Nostalgic concerts are definitely not a sign of relevancy outside of his fandom bubble.