Oh my this is some fun narrative setting bs.
Marner is hardly the first top 5 pick to be sent back because he was physically immatutre. Let's not try too hard to make this an issue worth worrying about. Because it's not at all.
Since 2010 Huberdeau, Drouin, Johansen, Ryan Strome, Sam Reinhart, Leon Draisaitl (eventually), Michael Dal Colle, Nino Nieds and Sam Bennett were all sent back to Junior in their first post draft seasons. That's exactly half of the 18 forwards taken in the top 5 since the 2010 draft. And you could very easily make the argument that some of the nine who didn't get sent down should have been sent down (RNH for example) and that Yakupov and Galchenyuk shouldn't even be including in the discussion since they had the luxury of spending half the season in other leagues beside the NHL because of the lockout (Yakupov in the K, and Gally in the O), which brings the number to 9 out of 16.
So frankly it's not only a stretch to say MOST of the elite players in their draft years begin their NHL careers, it's flat out wrong. I wouldn't even worry all that much if he was returned next year to London. It wouldn't be the first time a top 5 pick has been sent down twice. Florida did it with Huberdeau and it seemed to work out fine. For very similar reasons Marner was sent down this year.
And finally, it astounds me that this needs to be said AGAIN, but there is no rule in development. Not a single damn one. Development is not a one size fits all program that just happens. It's different for every player that goes through it. Stop trying to make it something that's a linear equation for every single player ever.