Of course I didn't say Yua was the first J-pop-to-porn conversion. IIRC there was at least one ex-AKB who had a full length JAV career, I think one only retired recently. She formally retired instead of the usual fading/disappearing quietly. Yua's JAV career isn't even 2 full years yet so it's still too early to talk about longevity.
But she is the first to make it a blockbuster success. Before Yua, celeb-to-JAV was "look how low/pathetic she sank". Yua was the first one to make some big money for the studio (and presumably for herself) so after her studio/agency put out attractive offers and some current and former celeb tempted to JAV. Perhaps to say, before Yua from anything to JAV was a downgrade. After Yua, for the low-end and/or fading celebs JAV could be an upgrade and success path.
Miko's still a newbie, so her direction isn't set yet. She still has her J-pop persona going so I am kinda wishy speculating where she could go, perhaps a whole new niche that no one has ever created before. Actually her SSNI-123 is way too rapey for me. I really really don't like r*** vids, I guess we are on the same page for that. I do like S&M vids, either dom or sub. (shouldn't need to explain the difference between sub and victim, but ... sadly JAV producers may not know the difference) I especially love idols who play dom roles in some vids and sub roles in other vids. E.g. Mao Hamasaki and Aimi Yoshikawa. Esp. after seeing Aimi in some dom roles, it makes it tolerable, actually very kinky to watch her r*** vids. Anyway Miko doesn't have to go to the S&M bottom, there's a range of mid-level themes that for some stupid reasons top idols (Yua, Shoko, Rion etc) wouldn't do. Ahh... good comparison would be Sora Shiina, or Atomi Shuri, imagine Miko do same kind of vids as them but apply her persona as minor celeb. Miko isn't nearly as pretty as Atomi Shuri, and mayne not even as young (looking) as Sora Shiina, but the magic dust...