@Bold, and Katakuri could survive an entire period of G4, actually attack it, and consistently avoid hits with his Observation.Except Luffy didn't push Katakuri at all until the end of the fight. At no point before that did Luffy actually contend with Katakuri power-wise save a brief moment when he went G4.
You mention Doffy can't win without Luffy's Observation Haki, but at no point was it demonstrated that that foresight actually helped Luffy at all during the fight as opposed to regular Observation Haki. Especially since Luffy was dodging Katakuri's attacks before he started seeing the future.
Doflamingo has enough power and speed to keep G4 at bay for 20 minutes and react to it. Katakuri has a higher chance of winning, but Doflamingo still has a chance due to having the same quality that allowed Luffy to win: High endurance. This is plus the fact that Doflamingo is in all likelihood stronger than Luffy, so he should be able to capitalize on this.
I'll give it to you that Luffy could dodge Katakuri's attacks back to back well before he gained advanced Observation, but that good execution still can't be said for his attacking. Not that I think Luffy's dodging was actually a thing he could do with all of Katakuri's typical attacks. It seems that Luffy was only dodging the trident consistently. Anything else pummeled him before his haki improved.
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I get that Luffy only needed to push Katakuri a little compared to what Doflamingo took, but I don't think Doflamingo is going to be pushing Katakuri at all is what I'm getting at. What put Katakuri down was ultimately a punch from G4 after taking multiple opposed to him just getting tired. As long Doflamingo can't damage Katakuri, he isn't winning.