I don’t know where people came up with the notion that there is no diminishing return in Fractals.
As it stands, fractals are not considered dungeons by the game. Rather, they are like personal story zones, where mobs are counted toward the world map diminishing return. Therefore, it’s noticeable and plausible that the same diminishing return from regular maps.
I would like to be proven wrong on any or all these points by developer comments, namely the people who made fractals, not community managers.