Obito doesn't need Rinnegan to beat Sasuke. Sasuke has no counter to Kamui intagibility. Obito simply goes inside Susanoo and warps him.
Obito is probably the strongest counter to Susanoo out there.
Even -IF- he couldn't get through it, Sasuke has no way to beat him. Obito can warp the Susanoo arrows ( or himself ), can't be affected by Tsukuyomi and can warp Amaterasu. Sasuke would eventually run out of chakra, whereas Obito's Kamui has not been shown to exhaust him.
As if that's not enough, Obito can use the Animal Path to keep Sasuke's Susanoo busy and even Shinra Tensei if somehow, by **** all logic, the Susanoo arrows would get to him.
Preta also absorbs Susanoo.
Sasuke has no way of winning whatsoever.