Zoro said he wants to become World's Greatest Swordsman, not defeat a Man named Dracule Mihawk
It's just that Mihawk is the Current Holder of that Title, but if Zoro surpasses him & there is still another Swordsman who is/was greater than Zoro then Zoro's Journey didn't end, it just means there is more he could reach
Zoro will Surpass every Swordsman & this includes Oden, Rayleigh, Shanks, Mihawk, Roger & even Ryuma ... etc
Just like Luffy who is destined to not only become Pirate King but also surpass Roger because he is gonna use One Piece & not simply find it (Roger wasn't the Chosen one), even though the Character he wants to defeat Last & surpass is Shanks
So basically both Luffy & Zoro want to surpass Shanks/Mihawk from Current Era who are Rivals & then both surpass Roger from Last Era & then surpass Joy Boy & Ryuma from Ancient Times. So both are destined to become Greatest of All Times