Imaro
Legend
But as far as I can tell it is mechanics first: we don't have a prior, in-fiction conception of how tough a 15th level fighter is, and then set DCs and stat up creatures that respond to that. We don't know how tough a 15th level fighter is until we see what s/he can do, taking certain mechanics published in the MM as given.
That's not fiction-first. It's mechanics first.
That's not true unless you are purposefully ignoring the same descriptions of tiers in 5e that are present in 4e.