There’s no reason that Saturn can’t be defeated here. It’s nothing like WCI and Big Mom/Katakuri.
BM couldn’t have been defeated on WCI for a host of reasons.
Half the crew wasn’t there. There was absolutely no chance that the first Yonko was going to be defeated without Zoro and the others playing a part.
Luffy was not ready yet powerwise. It would have been a wild jump for Luffy to go from Doffy/Cracker fights to going one vs one with a Yonko
Punk Hazard, Dressrosa and Zou were all arcs devoted ti building up to Kaido being the first one to be defeated. Big Mom had a couple of chapters at the end of FI and on Zou, and we went into the arc knowing it was just a side mission to get Sanji back.
Also, the Gorosei are not like the Yonko from an organisational point of view.
The Yonko are four (well, now seven) totally different organisations, made up of their own captains, crewmates, islands, allies, enemies etc. Each one is an entirely seperate thing.
The Gorosei are more of a monolith. They are all in the same organisation (as “Commanders” to Imu). They are all designed similar. They are all from Mariejois, they are all Celestial Dragons, likely to all have Yokai themed DF, all have names with the same theme. Beat one, there’s still four more to take on, and the one in charge of them all, Imu.
Luffy’s now at a stage where he should be able to fight anyone. Can’t see any reason why he shouldn’t be able to fight one of Imu’s commanders. Also, Saturn will be strong, but I heavily doubt that he’ll be stronger than foes still to come like Blackbeard, Shanks, Akainu.
And if he is, and he can’t be soloed- then he can be taken down as a team. All the crew is here for once, plus Vegapunk, plus Kuma and Bonney. If it takes all of that to take down Saturn, there’s you’re build up for the other four already set.
And you can make it even more hype if you want, by having them unable to beat Saturn and it requiring something suicidal Kuma bomb to wipe Egghead
off the map along with Saturn.
One thing’s for sure- the dramatic tension is never going to be this high for Saturn’s defeat again. We just had the ten chapter flashback building up why Kuma and Bonney hate this man and he should be defeated. This is us at peak “fuck Saturn, taks him down” emotional territory. It can only go down after the arc is over. If Saturn survives for another day, he will again be coming just one of five to be defeated. It’s only here that he can be the main villain.