Due to plot no jutsu, I think we need to accept the fact that sooner or later, Naruto and Sasuke will face Madara as a tag-team with no more help from the others. With that in mind, both WILL need to use natural energy.
We all know Naruto can use it. So we can leave him out of the discussion.
Now lets focus on Sasuke - To see how and why plot will make him learn sage mode (very likely to be perfected dragon sage mode):
1. Orochimaru's experiments shows Sasuke has the potential to be better than Madara. Note that when Oro made this comment, he already knew Madara has Rinnegan and Hashirama's DNA. How will Sasuke top this? Obviously it will need to be something like a little DNAs of Oro, Kabuto, Karin, Suigetsu, Jugo and Hashirama.
2. The dragon sage mode which Kabuto displayed is imperfect. Kishi first did it with Jiraiya where we were showed how an imperfect toad sage looks like. Now its time to show us the perfect dragon sage mode. Agree? I do want to see it for my self.
3. In terms of plot, it would be uncool for Sasuke to keep utilisie Jugo's ability to attack Madara. That would equate to Naruto, Sasuke and Jugo VS Madara. Sounds totally wrong. He needs something on his own.