Why does Luffy need a backup against Admirals soon? He is probably in his prime when he is about to fight Akainu, same goes for Zoro (Fujitora), Sanji (Kizaru) and Jimbei (Ryokuygu).
Even if we count Im, CP0 and the Gorosei to that, there will still be the revolutionaries along with Dragon and the Strawhat Grandfleet who are gonna teaming up with Luffy. So no need for Yonko alliance bullshit