Why is Kishi stalling on giving Madara his second Rinnegan back?
Its because it's the greatest power up that he would ever receive.
Yep, greater than getting the Juubi, greater than getting the Shinju.
Then, Madara with only Double Real Rinnegan>>>>>>>>>Juubito by far.
That is, if Madara had to trade away his Juubi and Shinju for the other Rinnegan, he'd do so in a heartbeat.