The marines are actually going ON the island to retrieve a bunch of shit (York, Punk Records...) so the notion of destroying the island itself with a massive buster call is either canceled or postponed (unless Saturn really is in a hurry and doesn't mind blowing up the marines who stepped onto Egghead to have Bonney and Sentomaru surrounded, as long as it kills Vegapunk and Luffy, any sacrifice is acceptable?)
And as for Luffy's side of things, besting every single marine on the fleet + one of the elders (which, again, nobody on Luffy's side even knows that Saturn is at Egghead at all) was never stated to be a stake or goal in this arc at all. The only task and focus is to escort Vegapunk and every ally to Elbaf safely. The struggle here is that Luffy has to protect pretty much everyone all at once, it's like playing a Pirate Warriors level where you have to protect every SH all at once as they run around the stage, except in this scenario Luffy is faced with the likes of Kizaru and the genius Lucci who have proper IQ instead of generic enemy officer AI in a videogame lol. And in this story, Chopper and Nami can't magically beat such opponents by grinding enough levels either.
Sure it might seem manageable now if Luffy has extra hands in Zoro and Sanji to help keep Lucci at bay and protect Bonney respectively, so he can focus on zooming all around the place and fight off Kizaru exclusively. However it's gonna be tough if the CP0 agents and seraphim somehow get freed. It's gonna be tough when it's revealed as soon as next chapter that Saturn has a firm hold on 50(!) advanced pacifista units, and when the Blackbeard lieutenants come around to create mayhem. Sure everyone is speculating that they're here to kill Kizaru at the last second and steal his fruit, but come on now. It wouldn't make much sense if their whole purpose in this arc was to indirectly benefit the protagonist and his friends as opposed to just being even more of a thorn in their side (unlike Big Mom in Wano cough)
The deeper we go into this arc, the more unsustainable this whole operation will be. Luffy barely has any capable fighters on his crew, and the likes of Stussy, Bonney and Sentomaru are either very weakened, or outright eliminated until further notice. The arc will either end with Luffy getting overwhelmed and captured (something that can also be qualified as a huge incident), OR another faction coming to cover Luffy and his escape, or perhaps just making a mess of things for everybody involved if said faction isn't exactly a good one, who knows. As it stands, unless Kuma or even Caribou somehow save the SHs miraculously, then even with those guys around the SHs are just ridiculously undermanned.
The Egghead incident might not be about Luffy beating a whole admiral and elder or taking them hostage, and that's okay. Morgans embellishing tf out of Luffy's actions in a given arc has never harmed the story, it has only bettered it, so why would it start being a negative now lol
It's the final saga, let the bird go nuts, Luffy will have great accomplishments down the line either way.