Has he taken on well-known swordsmen and beaten them? Some of his best feats just include pushing away a blind admiral and... a long-ranged attack that sliced a huge stone? Luffy could be stronger than an Emperor and still not even be in the running for the fifth emperor or successor if he doesn't fight someone on a similar power level to them and win.What is a proof? Wss and yonko title? He cant be close to them without titles?
His swordsmanship will never change.
His physical strength seems to be good enough to clash with top tier Fujitora and push him back through gravity.
His CoA seems superb to the point that he might create a kokuto this arc which not even Shanks has.
So, what's actually missing?
Likewise, Zoro could be stronger than Mihawk, but it'd all mean jackshit if he doesn't actually prove it.