Why people can't FATHOM the possibility of Megumi becoming top 1

I agree that Yuta and Yuji may have more potential, however, we didn't see all the potential that Megumi has, so why people assume so quickly can he NEVER could reach even special grade?

Narratively wise, is pretty implied that 10S is the best CT of the manga, Sukuna wanted to free himself of Yuji and used Megumi as a vessel, not Yuki, not yuta, if not Megumi. Gojo said that Megumi could surpass him, and that's what technically happend when Sukuna possesed his body. I'm fact, i think (maybe i'm wrong) that the 10S is the only CT technique that causes political conflicts in the sorcery system, specially beetwen the gojo and zenin clan, so it plays a huge role in the story. Even then, among all the students of Gojo, i think that Megumi is the one who put more confidence saying that he could surpass him, and the history makes an interesting paralell to this in his fight with sukuna.

And feat wise, his potential really depends on how far his CE gets, when his CE increases, the power of the shikigami does tho. The difference of the divine dog in chapther 6 and chapter 271 is HUGE, or the difference of sukuna NUE and Megumi first Versions of NUE, so it's easy to assume that stronger Megumi gets, their shikigami gets stronger tho. He can basically extend a fight until mahoraga adapts (we know that Megumi is good doing this using as example when he extended the fight with 3F sukuna), or even better, he can fuse mahoraga with other shikigamis, like the rabbits, imagine a full army of mahoraga rabbits. Megumi ins't even at half of his potential, Gojo stated that the thing that stopped him from reaching his full potential was his mentallity, and even then he was likely the fastest sorcerer in learn a domain expansión (incomplete atleast), with a right mentallity he can learn DA, RCT, barrier techniques, simple domains, etc. And talking about his domain, can we talk about how broken that is? He literally can summon a Horde of mahoraga's at the same time while his opponents is using CE in their feets to not fall into the abyss of Megum shadows, just give it a barrier, or even better, an OPEN BARRIER.

I understand your doubts, i don't even like Megumi, i hate it as the next guy, but it would be foolish of me don't recognizing his potential as a sorcerer and how the story tried to show us how broken the 10S are.