@jestervae
What you call a mistake is just the point of Saitama's character : being BROKEN (the word used in the manga to describe S-Class heroes), in other words, unfitt in his own universe because he can't be placed anywhere in the scale of power. He is considered as an error, an anomally from the rules of his own universe. Him being placed outside of his universe, he can see things from a distant point and be really pragmatic, a thing that wouldn't be possible if attached to the world's rules. (Not having the same common sense as all the living beings of the universe you are living with). To say it more simply, it's like living in a manga or an anime (or any fictional story from a book or a movie) and having the same thoughts as the reader or the spectator. (Not just at the beginning of the story like all the isekai protagonists who are saying : "oh, it's like a RPG!" and then they adapt their thoughts and atitude to their environment.)
OPM does not have the tension issue that Solo Leveling has. In Solo Leveling the tension is crucial, fundamental. Why ? Because everything revolves around Sun Jin Woo (which is normal, since he is the main character), including the power scale ! All the characters from Solo leveling are stronger or weaker than him. Until now, nothing original... However, our friend Woo can climb a magic scale, the higher he goes, the stronger he becomes.
But there is a huge problem with this system : what heigh will reach the top of the scale (and for what reason the scale has to end there) and where will be placed all the side characters and vilains.
What you are refering as lack of tension is the insufficient distance separating Woo and the vilain and a ridiculous gap between Woo and characters that were said to be very strong at the beginning of the story and are progressivly becoming insignificant as the gab is widening. And as if it wasn't enough, the vilain has to be stronger than the previous one...
In One-Punch Man, the constant tension is not as needed as it is for Solo Leveling.
Simply because of what I wrote at the beginning, Saitama doesn't belong to the scale. He can't be placed on it !
Moreover, this situation allows Saitama to interact with the other characters without any discrimination wether they are on top or at the bottom of the scale. He speaks to a C-class hero the exact same way he speaks to a S-class hero.
(that is the exact oposite of Sun Jin Woo, who is determined by the rank he belongs and hang only with hunters that are not too far away from him in term of magical power (the S-rank and the A-rank hunters).
In OPM, the 2 points that compensate the absence of a real main character are the following one :
- having a lot of great characters ;
- having Saitama interact with them.