Yeah you're probably right, mate. Not very plausible considering the vast power differential, and even EOS might be too much too soon for Coby - even though he's been trained by Garp and shōnen progression isn't always linear (e.g. Coby could power up by inheriting Aokiji's Hie Hie no Mi if the latter is killed). It's just that I can't think of a better character to defeat Sakazuki in terms of both — the narrative (apart from Luffy and Sabo, obviously), considering Akainu and Coby's previous impasse, as it were, at Marineford:
And the symbolic change of guard - with Coby emerging as the Sengoku/Garp type figure of his generation by overcoming Akainu and ushering in a new era for the Marines. And that includes Kuzan - mostly because even though he's also philosophically opposed to Akainu, he isn't as significant to Luffy's overall storyline (or the future of the One Piece universe) and wouldn't be as big of an underdog (with a point to prove) as Coby.