You're completely missing the point about Saitama. He's a parody of overpowered shounen characters, and so his power is being stronger than anyone else. And the story is written with this in mind, giving secondary characters like genos their chance to shine, even if at the end of the day Saitama takes care of the stronger ones. Also, Saitama didn't just suddenly become "cool". Before he became op as fuck he struggled against his enemies and could actually receive damage.Yeah, a lot of people like Onepunchman and Saitama, even though he became cool by basically just training every day, but a lot of people hate Kirito for being cool, even though for a 19-year-old guy, he went through too many life-and-death situations before becoming what he became. And the support of many friends, do not forget about it. Double standards.
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Basically, OPM is a parody of others, never taking itself seriously and focusing on being fun instead, like Konosuba parodying other isekais and becoming one of the best isekai in the process. On the other hand, Kirito is a character who automatically erases most of his enemies and earns the love and admiration of many women in a setting where he's supposed to struggle. And it's this lack of struggle that made many people grow tired with the character.