The Roger pirates disbanded, and Shiki and Garp (which Oden did not even know about) would have even less business coming to Wano. So it all boils down to Whitebeard. Now we know that due to Wano's status as an isolated country, even Whitebeard found out only many years later what happened.
Now the question is, assuming Whitebeard was stronger than Kaido, would there be a point for Oden to assume that Whitebeard will invade Wano and start a war with Kaido? I doubt that's something Oden would have considered, hence he never even mentioned Kaido's presence to Whitebeard or the Roger pirates.
Oden wanted to treat it as an internal affair. Or else, even if Oden assumed no one could've taken Kaido 1 on 1, he may as well had mobilized both Whitebeard + the remnants of the Roger pirates (Rayleigh, Gaban & co.). Obviously, Whitebeard joining hands with Rayleigh and Gaban would've ended in Kaido's tear down and that's not even up to debate.
So the most plausible answer is: narrative. He anticipated a great war 20 years ahead with key players making it to the new world, invading Wano and taking Kaido down. And indeed, the worstgen rookies managed not only to take down Kaido but taking down two Yonko. And I think we all agree that this alliance would still be inferior to a team up of Whitebeard, Rayleigh and Gaban, who would have all been happy to either aid Oden, or avenge him afterwards. But then again, it was Oden treating this as an internal Wano affair and putting his faith in those figures who will come 20 years into the future and throw the world upside down.
So I think over-analyzing this from a power level perspective makes little sense, because again, even if you put Kaido above everyone else 1 on 1, a team up of the Whitebeard and Roger pirate remnants would've stomped the beast pirates. Ask yourself why Oden didn't even want to call his buddies to dethrone Kaido.
		
		
	 
 
	
	
		
			alliance would still be inferior to a team up of Whitebeard, Rayleigh and Gaban, who would have
		
		
	 
 How come, man? The mighty Whitebeard needs to form alliances with two or more top tiers just to take down Kaidou?

This isn’t about prophecy or assuming/supposing that someone would go to Wano — Oden’s words are clear and direct. It’s about who could or could 
not take down Kaidou 
at that time. And according to Oden, no one else was capable of doing it.
Everyone knew Kaidou was in Wano. Gaban didn’t go there because he was afraid of him. Whitebeard found out a few years later — yes — but he 
did find out, and he didn’t start a war because he feared losing too many men. Kaidou used to make expeditions outside Wano, recruit pirates from beyond it like the F6, and even left Wano to go to Marineford.
When Oden and Kaidou talked about who could take him down, both of them projected it into the future and ignored all the characters from the present. Technically, both Kaidou and Oden were saying that 
no one back then was capable of doing it.
	
	
		
		
	
	
We have Whitebeard knowing everything and avoiding a fight with Kaidou, Gaban knowing everything and avoiding going to Wano because of Kaidou, Blackbeard calling Kaidou a terrifying monster, and finally Green Bull admitting that Kaidou was a deterrent to external enemies (including the Marines).
But according to you, the reason no one challenged Kaidou in Wano and took him down earlier is simply because no one would go to Wano for two decades — when the manga clearly shows that they 
didn’t go to Wano for two decades precisely because they were 
avoiding Kaidou. So Kaidou can’t be blamed for people not going to Wano out of fear of him.
They stayed away out of fear — not because of a lack of information.