Many people seem to believe that Luffy should be much stronger than the other Supernovas because he received training from an expert teacher like Silvers Rayleigh.
While this is true to an extent, there are reasonable arguments as to why the likes of Kid and some others may be able to match him, or come close.
With what we learned from Rayleigh, intense high level fights are the way to elevate your Haki to the next level.
However, the New World is an unforgiving sea, so rushing in to many battles is a high risk high reward situation. The risk is severe injury or death, but the potential reward is exponential growth. We know from the Vivre Cards that Kid threw himself in to many fights, lost an arm, has numerous scars and could’ve lost his life as well as his friends lives. But since he still lives, he’s likely reaped the rewards of that high risk situation and has grown significantly. At the same time, he’s also probably trained in between fighting.
In contrast Luffy’s training with Rayleigh was a low risk high reward situation. The tutorship of Rayleigh was a huge advantage that allowed amazing growth, but at the same time it probably lacked the danger level and intense battles that Kid and the other Supernovas may have had. He fought many strong animals, but we can see that Luffy had no additional scars post timeskip aside from Akainu’s so his life wasn’t ever at severe risk.
The reason that Luffy wanted training was because he wanted to reduce the risk of his crew mates dying by growing stronger to be able to protect them, as well as allowing them time to get stronger as well. Whereas Kid took the chance. So, if eventually we do find out that Luffy and Kid are close in power, it doesn’t mean that Luffy’s training was useless as some people have said, it’s just that Kid had a more risky but just as effective alternative.
Looking at things logically, Luffy will have rivals close to his strength in his generation even at the end of the story.
Roger himself had Whitebeard that was equal to him, as well as Shiki and Big Mom who stood close behind.
*Theory by Titan906