Warning: Spoilers for the anime and manga.
When Sir Nighteye is testing Deku for an internship at his agency he plainly states that, “Miro should have been the one to inherit “One For All.” He is completely wrong about this.
When he looks at Deku he can only see his quirk or rather a lack of one. Which is to be expected in the society of My Hero Academia. A persons worth is judged by what kind of quirk they have. So when he finds out that One For All is passed onto this quirkless creature he cannot figure out why or what All Might saw in him to give this amazing power to Deku. While there is a boy who would be suited perfectly for taking All Might’s power and that is true.
If you looked at Miro on paper he is the perfect candidate, especially on how you theorize One For All work on his body. He could use at least 20% of the power right off the bat as he has trained his body not just his quirk for years, the implications are that he would be like a mini All Might and that’s the problem.
Society has learned that they cannot rely on one person to be the pillar of peace and are looking for ways to reform their current structure by creating small hero teams to take on this burden instead of one person, if you only have a singular pillar holding up the world it will crash down once it’s removed but have many and although it will tilt it will never fall. In the book of “The 8 Laws Of Change by Stephan A. Schwartz he describes the best way to change society is by non violent means and small, individual choices.” The current society is changing and with knew foundations you must rip up the old ones.
If Miro took up All Might’s title this would continue the stagnation within society and nothing would change adding on to the fact that Miro is not that bright.
While Miro is not as dumb as a rock he his no strategist and the only reason he is in the top 3 students was because of his quirk not his mind. To be the symbol of peace you need brains in your head, All Might may not seem like it but he is actually quite bright. The time that Miro was training to use his quirk Deku was studying hero’s and their strategy’s for taking down villain’s and in return created his own.
While Deku’s progress with One For All is going extremely slow he is picking up the pieces and putting himself together to forge his own identity.
Miro and Deku share many characteristics, just Miro exhibiting them more strongly. He is everything that Deku is not, self confident, charismatic and has extremely creepy eyes that give me nightmares coupled with a straight face that makes me want to scream in terror.
Miro is what I call a classic hero, a person who was not the underdog for long, while he did struggle with his powers he overcame it and in turn became one of the most powerful students at U.A. But Miro was born with a quirk meaning he is significantly disadvantaged unlike with Deku who was born without one.
When Bakugou was attacked by the Slime villain all the hero’s were standing around waiting for someone with a better quirk to arrive to deal with the situation, if Miro with only his quirk had been there he would be fruitlessly trying to combat this slippery villain while Bakugou is slowly choked to death as he can’t break him free.
In this society people have a over dependency to rely on them using it as there only form of fighting except for the few who are cannot do this. With the people who don’t have a fighting quirk they have to get creative, making them better hero’s as this open their minds to develop new ways to handle the situation before them and don’t give up just because there quirk isn’t suited for the battle, they find their way around it. While Miro understands this concept he is early Naruto in his battle tactics, punch them till they’re on the ground as it were and that is not a good strategy when you’re faced with an enemy that negate your quirk and in this world where villains are smart with extremely dangerous quirks his chances when faced with a villain that is stronger and smarter than him when he’s all alone, well the odds are not in his favour. He will be essentially repeating the same cycle that All Might started. Continuously sticking to the same foundation as his predecessor not knowing that the pillar of peace is a faulty one until it is to late.
For society to change so does it’s heroes otherwise there is nothing left but stagnation. Sir Nighteye can only see how Deku’s not like All Might and not the future of how society will be shaped by this boy and how he will handle this new world order that society demands from the hero’s. But that’s expected for a guy with foresight not future sight.