There's plenty of people left that could be a challenge to Luffy. G5 "overcame the Strongest Creature in the World" out of like half a dozen different caveats. It wasn't 1v1-No-Items-Final-Destination, Kaidou went through a whole-ass gauntlet- including Luffy himself several times- before willingly choosing to take Vajrang Gun head on even though he didn't have to. Kizaru showing up is a big deal, especially considering I and a lot of people consider his logia to be the strongest in the series if he ever chooses to go full out.
People are vastly overestimating Luffy now, which is funny considering where things were last year. He's extremely greenhorn with his powers at this point- awakened Luffy fought for a few minutes with Lucci, and immediately powered down to old man mode. Lucci awakened, while taking horrific damage that was "making him lose consciousness (like in the old days)", powered down and was completely fine. Kizaru has decades of experience and is frankly a huge wild card in this conflict.