Your example of the MacBook Air is invalid because the MacBook Air of today isn't priced principally on performance -- it is priced mostly by build quality and performance/watt/kilogram. Never before have computers had a better performance/watt/kilogram profile. You have to consider the whole package.
The argument that there is less need for performance is also flawed. It is not that humans have less need for computation, it's that the distribution of computation is changing. As mentioned before, a lot of computation is moving to the cloud. In many cases, this is simply the most efficient place for it to be; instead of having an abundance of computation sitting mostly unused on a laptop or desktop, computation is becoming a service where it is used on demand. Think of the computation required for cloud services like speech to text processing or Google Goggles. Most of the computation is farmed out the servers in cloud and only minimal.
I'm not even sure I agree that the decrease in local computational needs is a long term trend. For one thing, migration to the cloud can only continue as long as network bandwidth keeps up with cloud data demands. If network bandwidth is not able to keep up then I predict we'll see local computational needs spike again.
Even with some computation siphoned off to the cloud, AI will create a whole new class of applications for even local computation because it is so data intensive. When the next crop of AI applications emerge and become more common, the early algorithms will probably be inefficient, which will itself cause a spike in computational needs. The applications are here yet but there are many (including myself) that are working hard to change that.
I think that the market for computing power has a looong way to go before it taps-out what the market demands.
It looks to me that almost everything you listed could be done on the computers of five years ago. It's all nothing but software improvements. Existing hardware is good enough.
I'm talking about economics. I'm talking about what people are willing to pay for.
The customer would have loved for it to go faster, because then it would have looked like magic.
In 1998, it would have been nice if I had had photo-realistic computer games, but I patiently made do with what I had. Today, people eagerly shell-out money for hardware that can run this year's even-more-photorealistic version of Madden Football.
I think the demand for more power is definitely there.
Graphics hardware companies like nvidia/amd will continue pushing the envelope though
Kurzweil also ignores economics. The advance of technology is driven in part by economic forces. Computing power may stagnate not because we have reached physical limits but because present-day computers are good enough for what 98% of the market wants.
I see this trend developing. If anything, the trend in consumer computing is toward less powerful but lower-power and more portable computing devices. My current laptop -- a Macbook Air -- is actually slower than my previous laptop. But it is more portable and uses less energy. And it does everything I want. I don't need more power right now.
The only areas driving the performance end are gaming, high performance computing, and high-capacity data centers. How long will those go until they too are basically satiated?
We've seen this in other areas. The envelope for aviation maxed out in the 1970s with things like the U2. Space flight seems to just now be emerging from a long coma with things like SpaceX, but on closer examination SpaceX is just reviving 1960s ideas and doing them at a lower cost with modern control systems and materials technology.
My other reply about energy deals with supply-side limits to growth. This response deals with demand limits to growth.