Obito isnt winning unless he has Rikudo Chakra and Bijuu. He already admitted Itachi was the reason he was at bay and Yata can block Kamui since Kamui is also an attack...Obito wouldn't need to do that. Obito would walk past the yata mirror and totsuka, straight into the susano and neg diff Lard Itachi there and then.
Also Itachi can purposly let Obito suck him into Kamui world and then he would destroy Obito since he cant phase in the KW