The thing is most people who have a need for a laptop, don't have a need for a beefy laptop. Its the form factor they care the most about. Sure you and me need something more beefy. I have emulators, and simulation programs, and virtual machines running. My brother on the other hand has 3 spreadsheets, a word doc, and about 5 tabs open in chrome. A smart phone could probably handle his workload. I would take a guess that the average computer user has more in common with my brother then myself. To them there's probably a lot of value in having this all on one device.
A smartphone probably could handle his workload, but a CPU/motherboard design that was less constricted by size would be able to do it faster.
Also would your brother want to dock/undock his phone with his laptop all the time? I generally just leave my phone in my jacket pocket most of the time so if I want to go out of the door I don't have to hunt it down. Under this system your going to have to go find your laptop and unplug it before you can leave.
Under this logic it would have made sense for Apple to launch the iPad as simply a "big screen" dock for the iPhone. Instead they launched it as a separate device with faster hardware.
I predict that tablets will start to become far more powerful than smartphones in the long run as demand for high end graphics will increase whereas smartphones will become more optimized to reduce power usage.
You are trying to shove the future into a present-day sized box.
If things were working correctly, there wouldn't be any need to take your phone out of your pocket to have it drive the laptop.
And the laptop needn't be useless if the phone isn't around, it should just be able to advertise its screen and keyboard to the phone, so that a user that wants to push text at the phone can use the laptop to do it (without messing around with much of anything).
It really depends who far into the future we are thinking. If we have the technology to reliably stream high resolution video (of the desktop/UI) through the air with no noticeable latency at 120fps then all bets are basically off.
