@kevo_koma:
I thought the reason no one could get One piece was because anyone who got close would basically get annihilated by the Emperors
As Buggy stated: the strongest crew is always the one closest to finding One Piece.
The Yonkou is a direct result of Whitebeard, having no true equal after Roger's death, and his uninterest in One Piece (since he already had what he wanted+Roger told him what the Will of D means as well as what the Void Century was about; One Piece is hilariously obvious: you have to read the messages of the Rio Poneglyph as if they were one piece to decipher it completely).
To get to Raftel, a crew needed to be strong enough to not be challenged by any other. Given that, Big Mom and Kaido appeared in the picture as the most powerful pirates, save Whitebeard. But neither was strong enough to go to Raftel and not have someone intervene (i.e. Kaido tries to go to Raftel, Big Mom would intercept him and they destroy each other). And a great gap in strength is obviously the basic requirement. There is a lot more than that.
Again, Whitebeard protected territories out of friendships he built with the locals; unintentionally, that led to a land race with Big Mom and Kaido as well.
This would explain what Buggy meant when he said that Whitebeard "was the one closest to getting to One Piece"; he meant his strenght was almost enough to safely get to the last bit of the Grand Line unchallenged.
While Buggy most likely would not have any idea what One Piece is, Shanks might know which is why he has taken a similar outlook as Whitebeard in his position as one of the strongest pirates in the world.
Seriously, this is some very simple stuff to figure out. The same with Kaido and Shanks' encounter during the War of the Best.
Think of Yu Yu Hakusho and the last arc in which we found out the Demon World was ruled by three kings that never tried to confront each other. If two of them were to go at it over territory, it would leave the other one, at full strength, free to finish off the winner of the previous outcome (the winner will undoubtedly be greatly weakened).
The same goes for what happened in One Piece: Kaido only went after Whitebeard because he knew he would go to save his Commander. Whitebeard could not afford to waste all of their strength and resources trying to defeat another Yonkou and have enough left to save Ace…and that is not including the time factor. That is why Kaido jumped on that opportunity: he would be facing another Yonkou that would certainly not be fighting anywhere near his best and that goes for the rest of the crew as well.
However, Shanks intervened and the situation changed: if Kaido would have been stupid enough to engage Shanks, he would gain absolutely nothing whether he wins or loses (this time, he would be fighting a Yonkou and his crew; one that would be fighting at full strength on top of that). It would be the very definition of a Pyrrhic Victory.
And the above combines into having Big Mom fresh and alone in the New World with no one to oppose her towards Raftel. Kaido did the smart thing and withdrew.
I have heard all sorts of stupid theories (the SMILEs having time limits, Kaido being part of Roger's crew as well, etc.) when the encounter is extremely simple to figure out...talk about a lack of critical thinking skills.