Wow. I think we're all missing the point here.
Kubo is a fucking lazy ass cheap writer. I mean, first, Gin's bankai. It's just like his shikai but longer and faster. Hurray. I could just about swallow that. Then, Isshin using getsuga tenshou. He's not even bothered to think up new abilities anymore. He's just repeating them now. Sure, he might pass it off as father-son so obviously same ability or some bullshit like that, but I don't like it; that's cheap writing, a sign of laziness. I don't care what his explanation is now. It's reached a whole new level of shit-ness.
OK, I believe I can answer why Gennryuusai could've beaten Aizen. At least why Aizen believed he could've been beaten.
Also please feel free to correct me if you know more about this than I do and know more about Japanese culture.
It is to my understanding that in Japanese culture and myth the element FIRE purifies and is able to destroy everything. That is the basic belief and feelings towards fire. Just look at how fire is treated in other animes/mangas when people have it as their power.
So in this case Aizen believes that even with himself being fused with the hyogyoku he can still be burned/destroyed through intense flame like Ryuujin-jaka. The fact that Ryuujin-jaka is practically the FIRE GOD of all zanpakuto further proves the fear of fire.
I think your reading into it too much- The old man simply outdid Aizen in everything reiatsu, ability, experience and his Zanpaktou was overwhelming. If Urahara and company can hold off Aizen, condom wrap and all, without them so much as pulling a bankai then the old man could have owed Aizen. Simple as that.
Originally Posted by Yazen
I don't think they're holding Aizen off. It really feels like it's the other way around. He's just letting them attack for the hell of it.
For instance when you're playing a game and you gain a lot of levels, if lower levels monsters attack you, you either A) destroy them at once or B) mess around a little, maybe turn your back on them or let them whittle away at your health.
The answer B feels like that's exactly what Aizen is doing.