Jack hasn't taken any Ls at all, he's the most efficient calamity.
He's just a really stubborn unlucky errand boy, or should we say, a bad Luffy without plot armor. If an admiral captured Usopp on his ship and pre timeskip Luffy had to save him Oda would write some next level bullshit allowing Luffy to protect him no matter what. Now imagine a non-strawhat character doing the same but the admiral doesn't suffer PIS and has 1 former fleet admiral? If it was Who's Who instead of Jack, WW would have been buttfucked, sharing a cell with Doffy right now.
It's a miracle that Jack survived all the catastrophes he encountered intact and 100% healthy all the way up until now, that alone is a big W. No one will replace him.