Rooney has achieved incredible things. Being the top scorer for a club like United is no joke but has he fulfilled his potential? I am gonna have to say no.
Look, the answer of this question is very relative and no one is 100% right. But, I remember the very first years of Rooney and he could and should have been a Ballon d'or contender for at least a couple of times. He had lightning pace, incredible strength, good enough finishing and a very very good vision. On top of that, I used to feel he was a lot more competitive on the pitch. Apart from 2009-2010 season, I don't think we saw a consistent development in any of these strengths. I always felt his development slowed down after 2006-2007. SAF changed his position from main striker to a different version of number 10, maybe that played a role but I just don't think he never managed to establish a sustainable level like Messi or Ronaldo did. Yes, those two are on a different level above anyone else but Rooney was a better prospect than Ronaldo in their first two years.
SAF wrote in his book that, compared to a standard player, Rooney needed more time to recover. I am sure that played a role. If the lifestyle rumours about him is true, then maybe that is the reason why, but I am not a doctor. We always read about Ronaldo's unbelievable work ethic. We see the results on the pitch. We have Zlatan right in front of our eyes. Wayne never took of or never played on a sustainable level as those two.
I think, with his potential, he should have been a lot better but as I said, the answer of this question is very relative. At the end of the day, I am always grateful to his contributions. He is a legend for sure.