It goes back to Steve's cars vs. trucks analogy. There is no doubt in my mind that the iPad/tablet form factor was the car revolution of personal computing. They have sold like hot cakes and will continue to sell quickly all around the world. Their simplicity and price simply cannot be understated. With that said, the best selling vehicle in the States is still the Ford F-150, a truck. Laptops and desktops will still have their place in society and they will still be developed and maintained, even as future tablets gain more and more power.
Heck my smartphone has more power than computers I owned not even a decade ago, but you don't see me giving it up simply because of some specs.
Thing is, the gap is narrowing, from both sides.
I used to use my iPad 1 fairly regularly, because my old '06 MBP was getting slow, and worst of all it had terrible battery life so it could never be far from an outlet.
Since getting an rMBP though, it's the opposite. It's 4x as fast as my old MBP, gets great battery life, is lightweight, and has a fantastic display. Suddenly my iPad seemed slow (actually it still is, even after restoring) and with a terrible display (though still great battery life). Now my iPad mostly sits under the bed, collecting dust. I've used it maybe a couple dozen times since getting the rMBP in September last year.
TL;DR, we're at a point where "trucks" are getting great gas mileage, are practical, and can out-muscle cars.