A tablet is nice for reading in the bed or on the couch, but for working all day every day... you gotta be kidding me. How can you give up your 27" display, a physical keyboard and a proper mouse? That said, I'm a hardcore developer working with PDF, DOC, images, while doing photography as a hobby. I don't see myself giving up real computers, unless some sort of a direct brain interface is developed.
What happens is that for basic computer use a yearly upgrade is not necessary. Even I upgrade every 3 years or so, even though I'm running 10 virtual machines and shoot 36 megapixel images. The average person doesn't have to upgrade at all anymore. Unless they're still running Windows XP, which slows down completely after 2 years of use.