That is quite clear. Riethmüller was a very promising junior, then took a lot of time to find his way. Don't try to tell me you would have deemed him as "very promising" two years ago. Looking at his junior career your analysis would have been "why is this guy so far from the promises he once shown". That's exactly the point with Grotian. Things are not linear.
And how exactly do you manage to actually assess this "regression" ?
She's not dominating the WC charts but still register top-20 ski times regularly, nothing last year ensured us she would match with the fastest girls as of yet. Again, we could expect it but not consider it done.
In fact, if we compare apples and apples (IBU Cup Ridnaun Sprints) between 2023/23 and 23/24, we can see that she was 21st on the track last year, 11th this year, with a fall. Take it as you wish.