Luffy mastered his fruit just like Zoro mastered his swords. The fruit is part of Luffy so he learnt to depend on his power. Thats like saying remove Zoro's swords, he can't do jackshit, which is actually true lol. Atleast Luffy can read future, have advanced coa and coc. if Luffy didn't have his fruit he would have learnt and devise other way to fight, Luffy getting fruit is just a plot.
At the end of the day Luffy is stronger than Zoro, even if by a fraction.