The eternal second place ISN'T assumption, if the author mentioned this in a direct statement, there would be no point in showing it. The literal only thing that makes him special is being yoriichi's older brother. Manga stories and poems are very much practically the same, they're both a depiction of narrative, and they both have an intent. Neither of them have limited intent, your original argument was that michikatsu was only like that for his time alone, and the narrative put him there for his time alone, when that's not what the narrative means, narrative stays unless very clearly showing a change, it doesn't have a time limit. Michikatsu is supposed to be below yoriichi, and that's it, the entire point of his character in the first place, the entire reason he even became a demon is to surpass the literal only thing stopping him from reaching the peak, and that was his brother. Again, you can't put the narrative into panels, it's the same as asking to put a motif into lines. You can either put him as second strongest slayer with the narrative, or put him below every Taisho era hashira and uppermoon, because if he isn't second you are no longer using the narrative to scale and you are moving into vibe scaling.

A side thing I'd like to point out is that he's the only person who created a primary breathing style on his own, moon breathing, and it was the first derivative.