Until we see someone break through the perfect susano'o, that has tailed beast level chakra, you are just speculating that Hashirama broke through it... from what we saw, Itachi's susano'o blocked kirin. Kirin is possibly in the top 5 jutsu shown in the manga, transcending normal chakra and using nature energy. It's no stretch of the imagination to realize nothing can break perfect susano'o.
Sure, Tobi can dodge, but so can Sakura. Sakura isn't a good defensive ninja, she just doesn't let herself get it. There's a subtle difference between Tobi's 'defence' and susano'o.