similar to Harry Potter's "magic is mercury poisoning" fansplanation
Wait, I don't think I know this theory.
I'm sure I could dig up the tumblr post or whatever, but basically: In the Harry Potter universe, magic use is like mercury exposure. You're fine
initially, but when you keep doing it for decades on end, it takes a toll. This explains why every adult in the series is either
severely eccentric or downright evil, why wizard society in general is so small, isolated, ineffectual, and bizarre, and why children are the only ones able to reliable solve most problems.
It really explains too much for me not to assume it's the case.
Like I said, I think your definition of genre is too narrow.
The thing they have in common is the age group they're targeting. There doesn't have to be anything else.
For what it's worth, I think you're definition is just out wrong. Genre has always been about the nature of content of the work, and what you're describing has always been called something else, along the lines of target audience or demographic.
*shrugs* Shonen is about as much of a genre as, say, YA/Young Adult is. Which is to say it's regularly called one but probably inaccurately
I think the problem is that there's concentric layers of possible shonen-ness. The tightest layers would be classic battle shonen: Naruto, DBZ, My Hero Academia, etc. It's easy to draw really clear parallels with each of them.
A little outside that, there's a ring of sorta shonen but not quite the same: Sword Art Online, Jojo's Bizarre Adventure, maybe Attack on Titan. They've got a lot in common with core shonen, and a lot of the appeal is the same, but they've got noticeable differences as well- if Naruto and Bleach are siblings, Jojo is probably a cousin.
Finally, you've got the stuff that doesn't really have much in common with DBZ or Bleach other than intended audience: Death Note, Food Wars, I assume most sports anime. The similarities are kind of incidental, but still there when you start comparing them to works that aren't shonen at all by any stretch of the term. If One Piece and Jojo are cousins, One Piece and Death Note are probably a dog and a cat- not very similar relative to different breeds of either, but convenient for differentiating them from, say, a moose (Kiki's Delivery Service).
So:
"DBZ is shonen."
"Oh, when does the evil boss gloat about being super powerful, followed by the main character powering up off determination and defeating him with super lasers?"
"...every arc. Probably several times most arcs."
"Jojo is shonen."
"Oh, when does the evil boss gloat about being super powerful, followed by the main character powering up off determination and defeating him with super lasers?"
"...I'm sure that happens but I don't recall exactly when."
"Shokugeki no Soma is shonen."
"Oh, when does the evil boss gloat about being super powerful, followed by the main character powering up off determination and defeating him with super lasers?"
"...that's not a thing, unless lasers are code for the sheen preventing full visible nudity when everyone's clothes come off."
"DBZ, Jojo, and Shokugeki no Soma are all shonen."
"So they feature a young man with incredible powers fighting against impossible odds with his friends, and you normally discuss the show with young-ish men?"
"Well... one takes place in a kitchen and it's hard to tell on the internet, but yes."
Each tier has situations where it's useful, so each tier gets used sometimes. This can lead to obvious confusion when somebody tries to say "Death Note features some action and high stakes" and somebody else hears "In season 14 the protagonist cuts the moon in half with his eye lasers."