Downplaying Garling because he lost once to Berzerk Xebec ain't really fair, we're talking about one of the strongest characters to ever walk the One Piece verse.
I could tell you that Garling only needed a good hit to kill Xebec, since it was explained that being defeated/killed in Domi Reversi only takes you back in your former state without ulterior damages.
That aside, I get the matchup of Garling and Cavendish on an aesthetic level, but Cavendish never gave the impression of being someone with the potential to be an Adv. Conqueror, even less to be someone that can face and defeat an Elder in the Final War. Garling is portrayed as the most dangerous Elder in the long run, why should Cavendish be the one that kills him?
too many reasons to list here. i wrote a script for a youtube video but haven't edited it yet, not to speak of recording.
cavendish's sword durandal is named after the sword of roland, a mythical frankish knight. he fought saracens (old term for muslims). garling's hair and beard form a crescent moon, the symbol of islam. holy knights remind me of the templars, who were wiped out after they had been accused of actuall ypraying to baphomet (which was a corrupted spelling of mohammed) instead of the christian god. garling truly serves a false good, who looks like a devil.
oda once considered introducing cavendish as ace's past rival. had ace lived, he would have become one of the very strongest. pirate king level. cavendish should have similar potentiall then.
hakuba is much faster than any ohter character we have seen so far. should cavendish somehow be able to access that power at will, with advanced conqueror's slashes, not many should be able to stand up to him.