It is not built to last at all.
A Lenovo is built to take a spill head on and work. The air will die in a second.
A Lenovo is made to be dropped from 7 ft and keep trucking.. An air will be done for.
An HP, Dell, even any other Mac, when memory fails, you can replace it. When memory in your air fails, good luck BGA soldering new RAM onto it with a $20 kit from radioshack.
If you want to buy an air because it fits your workflow and you won't be tough on it, by all means, do so. But don't buy it under the idea that it has better long term potential than other laptops.
I agree that any machine, which solders all critical components on one logic board is quite prone to catastrophic failure/costly repairs.
On the other hand, I disagree with the praise you shovel onto other brands.
It's true that lenovo's have "fluid channels", so that liquids are supposed to run through the machine, but that's plain bs in real world scenarios. The most the machine is able to channel without seepage into internal components is about a tablespoon. Also, if you dip a glass of water on the table, chances are your machine is screwed, as it does not stand high enough off the table.
In my experience, real thinkpads (read IBM) are quite sturdy, but no way near indestructible, most lenovo designs are far less sturdy. Fact of the matter is that laptops simply are not baseball bats.
Regarding the OP's point: Sure, MBA's have some design compromises, which render them more prone to failure, while OTOH, they have less moving parts.
On the whole, I'd say: "MBA's have less component failures than MBP's, but MBP's allow you to replace the broken components easier."
RGDS,
