Mirio as a character represents the status quo itself. Even part of his own name can be read as conformity. Yes, he had some surface level similarities with All Might, but he lacked the drive to go beyond what already existed.
Early Mirio exhibited some of the same flaws as their hero society overall: to keep things running smoothly, he was willing to ignore the pain of the person in front of him. He crushed those who couldn't quite make the cut over inspiring others to act. In that moment where he abandoned Eri, he was the same as that old woman who left Tenko alone on the street. Perhaps he was a bit worse than her in that unlike Mirio, that random old woman didn't stop anyone else from trying to save child Tenko.
The purpose of Mirio's character arc was partially for him to overcome his conformist instincts and to take real risks to save people in need.
The Shie Hassaikai boss appeared in flashbacks throughout this arc criticizing Overhaul for abandoning his humanity for disregarding others to fulfill some higher goal. In a way, this arc punished Mirio similarly for detaching himself from his emotions to the extent he would ignore a child in danger to fulfill some kind higher goal.
