In my oppinion, SasukeMangekyo5 minds are really good. With EMS, Madara was able to read all of the text, and he started to know how Rinnengan and Sharingan are linked, and how to achieve it, after some time, he achieved it, but once you "Activate" the Rinnengan you can't turn it off like sharingan, but madara wanted to save his EMS for some unique abilities, so he sealed Rinnengan in Nagato, that he could do job for him, and Madara stayed in Mist village, after some time, Nagato achieved Rinnegan, and after death of Nagato, madara decided to take off one sharingan, and inmplant a Rinnengan. So when Madara have Rinnengan+Sharingan+Senju ( I mean Full full Uchiha and Full Senju ) he's almost at SO6P, but to achieve it fully, he have to get Jubi ( ten tailed beast ) and be jinchuriki, that he could be Fully Sage of the six paths, and for that he started the war. I mean this is how it was, it makes sense , and I think my theory is quite logic eh ?