Smoker was Luffy's first real threat he encountered, you can be sure they will have a final battle.
We havent seen him for 3 huge arcs so for all we know he has awakened his fruit and trained his haki under Garp and Sengoku and developed advance CoC that even Garp is impressed off.
Oda can pull anything out of his ass, but you can be sure this battle will happen and power scalers will hate it.
They won't have a final battle. Oda made it clear the moment luffy helped him out the first time and he let luffy and zoro go.
The next time we see him was in marine Ford. Where he looked impressive by that was only becuase luffy couldn't hurt him since he didn't have armament haki and because luffy was more focused trying to save his brother.
The next we see him luffy was casually laughing at the sight of seeing a dear friend smoker. He was no longer a threat to luffy.
He lost to law
Lost to vergo
Had to help the same law get his heart back from vergo for law to save them
The same law joined luffy
Smoker then lost to doffy.
The same law had to beg luffy in helping beat doffy.
Luffy beat doffy. Hell even before Gear 4 doffy didn't treat luffy the same he did smoker.
And the list goes on.
Smoker will get stronger but the most he will ever be is yc1 level. Now if oda gives him coc which he won't since he is already Shanks age and doesn't have these things then you may had had a case