East Blue about 3 years
Alabasta about 2 years
Skypiea about 2 years
Enies Lobby/Water 7 about 3 years
Summit War about 2 years
Thats 12-13 years for the first half to finish and some of those arcs even had small casts.
Fishman Island lasted about 2 years (this is still amazing for considering how little content was in fishman island.
Dressrosa lasted about 2 years and introduced even more plot lines that will need to be filled out more.
Now we enter Zou and learn more about Kaidou and others while also being introduced to another island.
Overall just to get some decent fights and stories fleshed out will take the usual time maybe even longer with a cast like this
and this is just the new world mostly without even including characters that might come up from different alliances the fleet will make with other pirates on behalf of Luffy basically. The cast will be stupidly huge in only a few more years and alot of those will have back stories.
I just don't see how this series is going to end in the short time of under 10 years.
fake edit: Doffy didn't really underestimate the Straw Hats, I think he underestimated their luck and such though. Like he had a plan for basically everything, but there was like something lucky happening for the straw hats at every turn that wasn't even part of their power. Like the Franky meeting up with the Tonattas and helping the revolution and having a way to enter and work their way to defeat sugar and such, stuff like that. Hell them even knowing what Sugar does before Doffy could use her powers against them and basically erase them from existence. That and Sabo/Fuji helping out the Straw Hats also.