We are getting to a point where PCs are starting to stagnent people need to upgrade their computer less and less while at the same time the tablet market is still developing. Most of the computers sold today will be working just fine and do the major thing required of them internet browsing and word processing (if they are properly maintained). We don't have to worry. A tablet purchased from apple today will be far obsolete in that five year span it may not even be usable with most of the apps released. People replace a computer every 3-5 years, a tablet every 1-3 years. That is the reason that the PC sells are slowing the tablet sales are raising.
Not sure if I get your point, but yes, tablets are generally evolving more quickly than desktop PCs, because mobile personal computing is the new cutting edge in personal computing.
This change is about wireless, shrinking silicon, solid state memory, battery technology, and now touchscreen for input.
Laptops and tablets are the new PCs. Plugged-in stationary PCs have plateaued. The personal computer now goes everywhere with us. There is pressure to make this a better experience as quickly as possible.
How can the tablet not be seen as just the latest evolution of the PC?
But notice that Apple is still seriously working on making desktops better, and a better experience. The new iMacs are revolutionary in a couple of ways. The new versions of the cinema displays are going to be amazing, and maybe the Pro as well.