Well, even though TU maybesorta was joking, you can't be too quick to discount Hatsune Miku. And maybe to an extent girls like Sashihara and Hashimoto Kanna are kind of stealth solo idols. But I guess you're really thinking about women who sing for a living?
I think these things are rather cyclical. I mean AKB has pretty much burned through its first couple of generations already and while Yukirin, Rino, Mayuyu, Paru and a few other "kami" remain popular, I think eventually the public will tire of the cynically throwaway singles that Akimoto's factory pumps out. Kokoro no Placard just barely cracked a million and Kibou Teki Refrain sounds weaksauce to me even by AKB standards. Plus, the more that AKB48 becomes "something your dad likes" the more that younger people will flee from it in search of something they can call their own. That's why Morning Musume had to become '14, so they could form their own fanbase of non-geriatric listeners, but even that couldn't happen until OG had pretty much faded from prominence. Akimoto's got a lot of OGs floating around. It's going to be hard for his empire to not eventually get crushed under the weight of its dried up old fruit.