I understand why people think Marner is eventually going to break through - on the surface, his playoff stats look decent, and he's simply too good a player to not eventually figure it out.
But as others have already pointed out, it's gone on too long at this stage for change not to be made. They've basically tried everything, from a goaltending revolving door, to changing everyone on the blueline minus Mo, to rotating the bottom 6 and trying everything from kerfoot to duct tape in the top 6 LW spots. And different coaches.
The only thing left is to try changing the core.