Establishing the difference between EMS madara & hashirama
The uchiha were given the
powerful chakra & spiritual energy from rikudou, while the senju were given the
life force & physical energy. Now what this dumbs down to is that the uchiha have strong chakra, while the senju have lots of it. They both carry a part of each others strength within them as per yin & yang but its not much to make a difference in this case.
Now with all of that being said, lets think about what makes hashi & what makes madara strong (not their overall strength but their most powerful techniques). So hashi has SM, which increases the power of his jutsus, shinsuusenju would be much smaller or hashi may not even be able to summon it without SM. So what does this tell us? Its says that hashi cant match madara's strength (chakra wise) without it. Otherwise he would get fodderized, no? I sure as hell believe so.
& now lets look at EMS madara & what makes him so strong. Madara has
nothing on his own which can aid him in a regular battle, he doesnt have any jutsus that increase strength or the scale of his jutsus because his chakra is already powerful enough. The only thing that would make madara as powerful as hashi would be something that gives him extra chakra, & what better thing to do that other than the one & only kyuubi. Kurama was madara's way of becoming even with hashi, he got fodderized without kurama, manga fact therefore its proven than he needs an extra boost.
(Note: About kurama, kurama has
never increased the strength of madara's jutsus, it was shown that madara can level mountains with his perfect susanoo sword slash, & kurama's tbbs do
nothing to hashi. He can catch them with his hands & redirect them, thats why madara uses bijudamma blades, because thats the only way he can get his tbbs to damage hashi. It has nothing to do with kurama's attacks, madara's PS blades are strong enough to cut hashi's mokuton & hands solo).
So as you can see, madara needs kurama in order to match hashi's
stamina, so he can last longer. & hashi needs SM to match madara's
strength, so he can fight on equal footing with PS.