In my experience, the problem with non-linear dungeon design isn't actually the casuals. The problem is the people who want to optimize. (Which, alas, does often include me.)
Think of Toto-Rak in 1.x. How many people actually made a point of opening up all the pathways and trying to fight all three possible diremites? How many people just found the shortest possible path to go get in one diremite kill, and only ever did that? I would bet you the number of people in group 2 was a lot higher than group 1.
In ARR, they seem to have learned from that, because they left in little side-objectives but made mostly one pathway. But how many people have ever bothered to go into the side-rooms (or down the side hallway on the second floor) in Haukke Manor to explore those spots in recent memory? How many people have done all the optional treasure room stuff in the Sunken Temple of Qarn when running it these days?
In contrast, how many people just grab everything they can between point A and point B, and then burn it down as quickly as possible in order to move onwards? (And again, I note I include myself in this group. For good or ill.)
We, as a playerbase, have historically demonstrated that given content with multiple pathways and possible objectives, we will maybe wander it once, and thereafter we will find the shortest possible route through and optimize it to be run as quickly as possible, while ignoring all side content along the way.
I can't really blame the developers for deciding to focus their effort on making dungeons that adhere to that philosophy.
