Gotta clarify something:
Rikudou Sennin didn't "beat" the Jyuubi. He sealed him within his body. That was the only way to stop him. It's like, when Minato fought Kyuubi. He didn't FIGHT Kyuubi. He just hindered him with a seal that postponed his rampage. It's not like he arm-wrestled the Kyuubi and slammed his hand through a table, so let's chill with the "zomg he beat da 10 tailz!!??11" It likely didn't go like that. I'm positive it didn't.
So anyway, there's that. Then there's the fact that you can't quantify the strength of the Rinnegan abilities. For example, they both can use Hungry Ghost realm's absorbtion power. So how are we gonna change this to Rikudou Sennin's favor? Allow him to absorb more than Madara can? Even if you wanted to do that, you'd have to comprehend the limit of Madara's absorbtion power before claiming such a thing.
I'll repeat that he'd win, but I'm just curious as to why everyone's so confident he would. I'm still iffy on it.
But seriously, I'm simply consering the fact that Rikudou Seenin had way more expericence and skill with his powers than Madara has. And he also has acces to a way greater amount of energy. So even they were to equal in everything else (conditional, conditional) madara would end up losing when the fight would be going on for to long since thr Rikudou can't really get tired considering who and what he is: the Juubi perfect Rinnegan jinchuuriki.
And that's only if we were to put the situation where they're equal in everything except for energy reserves, and but they're obviously not equal in so many areas, Rikudou Sennin having the the clear superiority of skills and expeerience in the Shinobi arts and especially with the Rikudou Rinnegan abilities.
I just don't get it why people asume RK has the same jutsus that Madara has? Yes Madara has a fraction of RK's power but that does not translate in any shape or fashion to having the same JUTSUS as him. Jutsus are learned... How do we know RK developed them?
Then why are we using Juubi for RK (the OP said we can argue with that). He LOST IT way before he died (aparently). If Naruto is going to lose the Kyuubi i am sure we are not going to use it for his future battles no?
Now if the Juubi is not into play we can't even asume the 'fraction of power" bit... Yes RK even with no that probably has more chakra but not much more. Just look at what Madara can spam.
Another thing Madara has wood element RK does not.