did dragon (revolutionary army) even fight them yet? nope
they prepared the top tiers just to fight an old yonko. and was afraid to fight shanks
if dragon is stronger than all of yonkos and at rogers level(Prime?!) then why is a commander of him so afraid of top tiers? if he knows hes got dragons back who is the strongest that ever lived in world craig!
an organised military institution has policy to enter a war only with overwhelming power (if possible), for example the US isn't comfortable to declare a full scale war/invasion without the ratio being 10:1, ofc in a crisis there are limitations but they are only physical, a professional military institution doesn't aim for ''just victory'' they aim for an overwhelming victory over the opposing faction ... it would have made absolutely no sense to not throw everything they have at WB, if you fight someone with a knife and have a choice you bring a fgin' gun and deal with the threat as soon as possible and with as little collateral damage as possible
sorry I love WB but the pirates got their ass whooped pretty good, and that was without Garp and Sengoku
and even if I rank Shanks second in power atm I am almost completely sure (lets throw BB out for this hypothetical) Garp, Sengoku, Akainu, Aokiji, Kizaru and the rest of the marines were just too much for Shanks to take on