If so, when?
I think ^ that's where opinions split - the timing. Although it wouldn't surprise me if there are football fans that say he didn't fulfilled his potential, which I may agree in someway.
Just seen this good video and it asked one good question "Did he truly fulfill his potential?". I don't think the vid answer their own question directly though.
OPinion
In my opinion, he did for 4 seasons.
Season 06/07 is when he fulfilled his potential, and he carried it on until arguably season 09/10.
Why?
Seasons after 06/07, I don't see any aspects of his game that are improving. None. That season 16/17 is the final season where Rooney showed "newer" abilities especially in the passing and temperament departments. I don't see any improvements anymore from him. After 09/10, I notice a decline in his physical abilities, then his all around football abilities too gradually declining albeit at a slow rate. SAF turning him into more of a CF role or stay higher up the field with a shift in system to compensate that are the right decisions to "maintain" Wayne's goal scoring abilities and getting the best out of him overall as he continues to decline.
Many will argued his goal-scoring ability and other abilities in the box namely hold-up, post-play, heading, poaching and fox in the box improved in 09/10, but I say that's a matter of switching into a roles that necessitates Rooney to do more of those than Wayne's incapability in doing any of that with the top qualities he showcased in that season. In simple words, he can already do all of those before, but he's playing different roles so it's rare to see him do those.
The unique thing about Rooney is how he's already too good when he breakthrough into Everton's first team and consequently quickly becoming a key starting 11 for both England NT but more so with us Manchester United. Below are how he is since the very start. He's ridiculously good this early before he's 20, it's freaky. Special player in a way.
Strengths
1. Physical abilities - already maximum top level. It's freaky. All-around have everything - fitness, strength, pace, speed, agility, aggression and acrobatics.
2. Football intelligence - a natural of the game. Instinct-based type. Almost always, his decisions makings (if he's in the right temper) is always right. He more often than not choose the right options - to pass or not, to shoot or not, right time to dribbles or not, etc. Rooney's natural ability to fit in all kinds of tactics (yes even LVG's philosophy) almost like a glove and to play comaptibly with any players are just impressively a proof of his natural football instinct intelligence. Note that it's not street-smart, you know the dark arts of football which Rooney is bad at using each time he tried.
3. Team-player - he's an extreme team player. It's rare for someone of his abilities to be this willing to sacrifice himself for the team and actually he doesn't mind playing "second fiddle" to other teammates, as long as that #1 player is a "worthy star player" eg. RVN, CR7, Owen and Zlatan, basically players he respect and regarded as that. Contrast this to players like Berbatov and RVP, I don't think Rooney is totally happy, though that's just speculations. From what I see, the way he play is totally different between the two categories. I'm not including striker partners whom he enjoy playing with here mind eg. Saha and Tevez.
4. Football abilities - all-around he's almost complete already, high levels, just need slight more improvements in few aspects. This is the major reason along with his top physical abilitiesHe literally can do everything, just need more games to get the experiences to improve those. I don't think he needs any coaching in any of the stated four aspects at all, unless you count anger management as coaching.
5. Mentality - winner, fighter and strong challenger. Just need a bit of maturity and control in temperament, right channels of emotions. I think this is why he's willing to listen well to top coaches and follow their instructions to the tee, because he's a winner, along with strong awareness of good football tactics when he sees his manager as top coaches. I think it's just with Moyes where he sort of "rebel" in a way he half-hearted follow the shit crossings after crossings instructions. This is why I felt a potential of Rooney improving in this aspect because he's willing to make the step-up early on and hungry enough to change career in a good career projectory that he made the step-up to work with SAF, and listen instead of complaining and rebel. I believe there are other offers at that time right?
Weaknesses
1. Temperament - as we know, he tends to let emotions gets in the way, and opposing team use this, they like to riled him up and before it often works, refs too had no choice but to book him. Working with SAF is the right choice and perfect next step for his career.
2. Maturity & Discipline - similarly, we know how Rooney were, on and off the pitch. This put his career and reputations at risk, thus even the treatment he got when on the field eg. refs tend to book him after any challenges, etc. He became much much more mature thankfully from working with SAF who insist on it.
3. Passing - I don't rate his all-around passing abilities from the start. There are many flaws. His long pass and cross are good enough but still in a way rusty and can be refined and improved. Thankfully he did later on, quickly too. Seeing Scholes performing it everyday in training is a sight to behold.
So what did all those show?
And that's it. Very few weaknesses from the very start and he's already that good. Need just a good refinement which only SAF can do, doubt other manager can. The different roles he play affected his goal-scoring statistics. If his roles is to play as the main goalscorer and play more inside the box instead of spending time moving deeper to help deeper, then for sure his goals will already be higher than how it is now. He started as a free roaming SS who literally go anywhere on the pitch. When performing that roles early on, I can already see his abilities in heading, poaching, goal-scoring instinct, hold up play, etc. It's all already there. He's just not doing it more, preferring to be in the build up and combative actions elsewhere than focus to remain in the box.
Stats and Contexts?
Note that Statistics doesn't always tell the whole stories. That's the limitations of stats, without contexts., it's laughably weak. Consider the different of roles he played earlier and later. It's starkly different and the stats backed those up.
Starting season 07/08, his injuries get worse and worse, thus his body started to get affected leading to first his quick declining physical abilities alongside gradual slow decline of overall football abilities. His goal-scoring abilities albeit declining is still at top level, thus the stats showed all that when his roles are more into staying up and score, but we know it gets worse in 16/17 onwards and at the top level, its no longer enough. His natural instinct-type football intelligence does maintained until now, hence he still able to follow and fit in well with the different tactics and maintain making good decisions makings and positioning, while still showing natural goal-scoring instincts. The biggest problem is at the top level, his physical abilities is not enough to match his natural goals instinct. He can only show it a lower levels nowadays isn't it.
Conclusion?
He first fulfilled his potential in season 06/07, and remain in his peak best until 09/10.
But you may define "potential" differently. I see it as a player maximum best possible or projected/forecasted improvements in total sum of football abilities (physical, intelligence, technical, tactical, etc), temperament and mentality. And no doubt Rooney max all three main categories of that quickly. Cristiano Ronaldo for me exceeded his potential in contrast.
