It's a shonen anime, the next villain is more powerfull than the last.
I think Doffy is very overrated by fanboys. Don't hate please.
that is not true.
Just look at part 1 alone.
Luffy was handicapped in the fights he could easily win against Don Krieg and others and got buffed against Enel and Crocodile. Meanwhile he ran into Aokiji, Kuma and fought in the war of the best.
Luffy throughout part 1 was basically consistently at 1 base level (with the exception of the gear boost) and the enemies were formed around him.
I understand what you're saying though because you're right if you would look at DBZ, Naruto, Bleach and Fairy Tail etc. But the only enemy in part 1 who pushed Luffy to his limits in a 1 on 1 fight was really Rob Lucci. That was an unrestrained fight from both sides. All the others were or either weaker or stronger than Luffy and Oda compensated for that.
And it's a bit too early to to call the same pattern in part 2 since Hody was just a demonstration match, Ceasar was still on the easy side. and with Doflamingo he got help from Law. You could argue that it is some form of handicapping the bad guy again.
Maybe in 3 arcs time we would look back and say that Doflamingo still holds up pretty well. But on the other hand if Cracker now has an answer to G4 it would be safe to say that the coming enemies will surpass Doffy.
But One Piece doesn't follow the standard shonen route
