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.