Damn, I just looked up how long I have been paying using https://payments.google.com/ . Looks like I've been paying for youtube music since October 2014. These grandfathered people must be really really early. :]
Same here, joined up when GPM was in beta. Still on the $7.99/month. I really only use it for YTM, so if they ever up my price, I'll cancel and use Tidal or Deezer.
I've been on the family plan for a long while... was before they redid Google Music and YT Music, but it included YT ad free and Google Music, so I did that and dropped Spotify around a decade ago. I watch so much YT content, it's been worth it to keep... Though I'm glad that Rumble is there and seeing some improvements in UX, still not nearly as good as YT, but getting better.
Only the earliest google music people are still grandfathered in at the insanely low rate. The rest of us have been "upgraded" to at least $14/mo.