They weren't nerfed, y'all just don't want to accept that narrative demands Strawhats to get that much stronger. Luffy's crew is going to be comparable to an emperor's post Wano for fucks sake.
EoW Franky and Brook would defeat Vergo (hope this doesn't go to discord).
Pretty much, it's just story progression, and it's not like it doesn't make sense, especially for Franky who has the most feats of the mid trio.
He could already defeat Senor Pink(a dressrosa executive) in base when he has a Shogun mode, which allowed him to dominate 2 executives(Baby5 and Buffalo) with much less issues, and thus placing his Shogun version at least a level above his base form
And the only people above the executives in dressrosa were the top executives(Vergo, Pica, Diamante, Trebol) and Doflamingo, when fast-forward to the Wano country, Franky was talking about building some anti-Kaido weapons back on Zou, so it's very likely he has stronger Shogun trumpcards now than before(maybe even visible by his new attack described in the spoiler).
So basically the progression is going crescendo, from defeating an executive in base, defeating 2 executives in Shogun mode, and gaining even stronger Shogun upgrades for the next level(top executives), with the top executives being equivalent to veterans / flying six in yonko crews, so it makes sense that current Franky could be able to hold his own now