Serge, I get what you're saying, and agree since seeing the pics of the rear storage box in the floor. I love the concept of that box and don't mind the floor being that high. I have some uses planned for that box.
The box is nice, but it's basically the same thing as having a box behind the current 3rd row seats. The only space that is really gained between having the current 3rd row in and the 'fold flat' ones is the slight difference between the seat thickness and the space the current removable rail system takes. That probably accounts for 4"/5" I would guess.
Without looking at the exact drawings of both trucks I would guess that 4"/5" is also about what is lost in overall height from the 2nd row seats to the tailgate between the new 'fold flat' configuration and the old configuration with the seats off.
Again, all of this simply means that GM's fold flat 3rd row is really more of a design trick that sacrifices volume from the current gen to get achieved.
The only reason it really struck me is because when we came down South, we stored all our furniture in DC and only brought short term necessities until we found a home. It was all packed in rubbermaid containers:
That was about half way done ... by the time I was done I had but an inch to spare at the top and the front seat was full, so basically I used up every available cargo volume space. If I had the SWB Esky I simply wouldn't have been able to carry everything. That said, I guess such circumstances are rare for most users ... but then again there have been a few trips back from furniture or big box stores where every inch counted, especially when trying to get stuff through the back opening (gigitty).
I think some folks are going to curse the fact that they don't have the option to have maximum space, but that most folks will gladly give that up for the fold flat option.