If oda wants (insert some shit here) to happen, then he would make it happen...
That argument is getting so over used, no shit it's his manga... power ups aside, I don't think you fully realize the gap between shanks and the alliance. To put things in perspective, it took the Law/Luffy alliance plus Happo navy's top, 2 other supernova (zoro, cavendish), and countless colosseum fighters just to beat the Doflamingo pirates. I don't see kidds alliance (from what we've seen) to be a greater force than what took Down dofly, someone who's noticeably weaker than Yonkou level and is scared of kaido. Even if Kidd, hawkins, apoo, and killer combined are enough to push shanks its not like his crew is gonna sit there, It's likely that shanks has a few members of his crew that are admiral level, or at least strong enough to make admiral take caution (Beckman displayed the later without even fighting).