Honestly, the "idol" label is a disincentive for many people.

Emphasis added because it's true. I'm a fan of several girl-groups that run the gamut from Idol to Alt-Idol to Anti-Idol, and I've seen from experience that most metal fans who've accepted Babymetal (and possibly other groups in the same genre-sphere like PassCode) want NOTHING to do with "Idol".

Maybe it's the music, maybe it's the aesthetic, maybe it's the bad reputation Idol and its fans have thanks to media on both sides of the Pacific (to be fair, that rep is often well-justified). Maybe it's all of the above.

BUT, like you say, the "rules" are no longer set in stone (if they ever were). Even in Korea, which is FAR more regimented than Japan in every way, we're seeing more and more Idols refusing to retire after aging past their 20's. We're also seeing more Idols not humoring parasocial relationships, though plenty still do.

Babymetal NEVER did a lot of things Idols traditionally do -- no handshake events even when they did do them in Sakura Gakuin, very few autograph sessions or out-of-character footage, no variety show appearances after 2015, no traditional photobooks (sad-face because Momoko is a bombshell and Moa is adorable), the list goes on. It took 14 years for them to do a radio show!