There would be much higher margins on a plain box A8 Mac Mini for $300 than a full rig with clam shell assembly, retina display, battery and keyboard. The 500 product you speak of would lose money. Apple sells a iPad which is just a screen for that much.
There was a time when cars did not need or have computers to operate, now they are musts. Same thing is happening to the home. They will be integral to the modern home. Control your all aspects of your home. Someone is going to fill that gap. Google and Microsoft are sure as hell going to try. There will be app ecosystems for connected homes. You have to look beyond the status quo. Your imagination is too instilled in the past.
The problem with your argument is sales quantity. Not many people want a mini computer. I want a portable one (laptop) and a full-sized one that can do serious work (desktop). A mini doesn't do me a lot of good. Given the sales numbers of the Mini, and the relative lack of PC-equivalents, it's obvious that most people feel the same way.
Your argument about the integrated home is silly. Why would this require an additional computer? I have a Nest thermostat, and it is controlled by my smartphone. You know, a device that you have in your pocket 24/7, and has an always-on data connection. I could also control it from my laptop or desktop, but I've done that maybe once or twice ever.
I find your scenario highly unlikely, at least for the next 10 years. On the other hand, Chromebooks currently make up 20% of the laptop market. Right now, today. They're also very similar to the A8 laptop that I mentioned above.