1. I don't think the straw hats are gonna even fight Shanks and his crew, that's if Blackbeard doesn't kill him before he gets the chance to see Luffy again.
2. Blackbeard and his crew are hunting devil fruit users. This means 2 things. One, they are getting stronger through battle experience from fighting and killing devil fruit users. Two, they are getting new powers thanks to the newly obtained devil fruits from the enemies they beat/killed. We already saw this with Shiryu getting the clear clear fruit and Catarina Devon getting the nine tails fox fruit. The battle experience and devil fruit powers they obtain are monumental, not to mention that they were already level 6 prisoners back in Impel down, so its not like they weren't strong before they even joined Blackbeard before the timeskip. So if the rest of the straw hats don't get battle experience and powerups now, then they are gonna get creamed when they face off against the Blackbeard pirates in the final showdown. Chopper was already struggling against brulee and robin's last real feat was low diffing Hammon back on fishman island. Robin's last real 1v1 fight was against Yama back on Skypiea.
At the rate they keep on going now, come the final showdown against the blackbeard pirates, not counting the monster trio and jinbe, both the mid trio and the weak trio are gonna get stomped on by the blackbeard pirates at the rate they are growing now if they don't improve from 1v1 fights against the flying 6 or other strong fighters from the beast pirates.