"For me, reading information on a screen is not working. Work is when you add value, so when you actually produce something with the information you have."

The total market for portable computing devices is growing, and the replacement of e.g. paper manuals by tablets might be one of the reasons. This is work by my definition.

"There is no reason for desktops to become more expensive, though. As long as they are built with mass-market technology, of course."

Many desktops in companies can probably be replaced by tablets/mobile devices. I'm in an open plan staffroom with 60 desks and 60 PCs. Many of my fellow teachers would be very comfortable with a tablet that could talk to a projector somehow and some good writing sketching software. We could halve the number of desktops easily, perhaps just have 10 on desks in the side of the room.

Multiply that all over and you have a reduced market in numbers for desktops.

PS: not sure why your comment is being downvoted, just depends on definition of 'work'. Mine is different.