^the thing is most of the people who were into Yumi 30-40 years ago are now too old and prolly don't go to Karaoke bars as much as people who are in their 20-40s who were young even when Ayu was still big. While I am sure Ayu's peak made a bigger impact on Japanese pop culture than both Matsutoya's and Seiko's did, let's wait another 20 years and then compare the results
But in order for any of this to make sense people would actually have to have more detailed statistics ^^
Also I don't think you have to know who Yumi is ^^ she has nothing to do with Ayu really, even though Ayu might've been influenced by her at some point. Sure as a jpop fan in general it's nice to know the history of stuff you like, but that's as far as the whole thing should imo go. It's not really a huge issue lol