I think I have a good theory behind the Bijuu's handing chakra to Naruto,

They basically created individual horcruxes inside Naruto to keep their essence or spirit alive, what I think will happen is Naruto will become the Juubi Jinchuuriki however instead of hosting the Juubi itself the Juubi's chakra will revive the Bijuu's with their essence, memories etc intact and from there they will go on to fulfill the Sage's prophecy with them merging into a more powerful being.