If it's true that Uzumaki/Senjus and Uchihas can get Rinnegan by getting each other's DNA in/on them, then there'd be rinnegans everywhere because they've been slicing each other up throughout history.
I guess the exception in both Madara and Naruto's cases are that the specimens they would've got their DNA from are special. Like, their best friends.
But yes, whoever ends up dealing with the Juubi once and for all is probably going to need rinnegan. Someone will have to awaken their own pair if Obito isn't converted or dealt with soon.
The part in bold is something I agree with. The Rinnegan is an important part of the story and I think that, if Naruto has a hope of defeating the Juubi, then he will need it. As for the other part of your response, see my response below...
There's no way that would be enough!! Otherwise Madara would have unlocked the Rinnegan a long time ago with his fights with Hashirama.
And shouldnt Sasuke have unlocked it by now if thats the case?
Naruto needs to get an EMS at the very least to have a chance at the Rinnegan. But I doubt the will get it, unless its a gift from Sasuke
Not necessarily true. Just because they've been fighting each other for a long time, doesn't mean that they've been exchanging DNA each time. You've got to remember that for DNA to be effective in this case, it must enter into the body of the person. Unless the Uchiha and Senju have been ingesting each other's spit or getting blood mixed into their wounds then it's not likely that there has been an exchange of DNA.
My point with the fight at the Valley of the End was that Sasuke's hand was in Naruto's chest. That means DNA could have been implanted directly into the wound without either of them knowing. Of course, this is just a theory of mine. I know it's not the most likely scenario, but that's why it is a theory and not a fact.