I was thinking about this 'loss of the desktop' on my drive into work this morning.
The "desktop" in computer usability implies to the user "This is where you do work - a virtual desk." It's the notion that's been presented for many decades of personal computing. As such, it's also what people are comfortable with because of its ubiquity.
Unfortunately, the average user has seen the desktop as "This is a spot where I save documents I think I need all the time and where icons keep appearing for programs I must be installing." There's those few that do this to an extreme degree (desktop is literally full), but there are probably just as few that try to keep an efficient desktop where the only things on there are what they are working on at that moment. This shouldn't have been unexpected based on how people treat their literal desks - trying to make storage areas out of a work surface.
Users also have lately been weaned off of "You must know where you store your data"/hierarchal storage practices in favor of "You obviously can't remember where you put things, so we'll just find it for you"/search indexers. We also know that Microsoft has been capturing statistics for their user experience as well, and I'll bet that they thought "Hey, people aren't really using the desktop for much. They seem to be or could be using the start menu quite a lot for getting to what they want."
It would seem that according to Microsoft the next logical move was to keep the desktop as we know it there, but put it in the background - don't make it the focus. Instead, they decided to make a glorified start menu that uses the whole screen and then some. Apparently Microsoft felt it's what users really need access to the most.
Working in user support we all know how many groans and grumbles come with changes to the ways things are done/have been. (Sometimes we are even the ones doing the groaning and grumbling). I don't think Microsoft wouldn't have decided to make such a change without knowing full well the backlash it would get. They would have to have sincerely felt that the general populace could get along fine without having the desktop up front and that the tile interface really is the next step in usability.
But it's their guess. The users will answer.