Shanks attended a wedding and Blackbeard went to get something before the marines do. They have not interacted with the Straw Hats at all. This new saga starting after Wano when the previous one is called the for yonko saga and the two yonko are still kicking is nothing but a marketing strategy.How do you know that? Shanks and Black Beard were on the move during reverie. We have no idea what happened during Wano.
Sabo stands no chance against Akainu. Sabo hasn't shown the qualities of a king and Akainu has an advantage when it comes to the devil fruit.
I also don't like people saying Blackbeard is the final villain. The pirate who finds the One Piece won. And it's not going to be Blackbeard. He'll either lose at Lodestar Island or Laugh Tale. The Celestial Dragons have ruled the world for 800 years with Im-sama at the top. Blackbeard unfortunately is just some guy. The WG has gotten rid of those types in the past i.e. Rocks.
No one is coming to Blackbeard's aid to fight the WG. There will be a lot of people coming to Luffy's.
And Luffy has a better shot to get some good marines on his side like Garp, Coby, Smoker, Fujitora and Kuzan. Those poeple are not joining Blackbeard.