1) The tablet market (the real one, you know, the iPad, not windoze tablets or early android garbage) is still relatively young. In 12 - 36 months I am fairly confident the ratio you cite will be in complete reversal. Just wait and see, particularly as android tablets get not only cheaper but also better.
2) Your perspective as a developer makes you uniquely unsuited to evaluate the average white collar worker. You are simply not one of them. The example you cite - an exec checking email in an airport (or perhaps reviewing a spreadsheet, marking up a presentation or contract) is *exactly* the vast majority of the professional working world today. And they can - and will - increasingly rely on the thinner/lighter/good enough capabilities of iPads and other tablets.
3) The convention you cite - of sitting at a desk across from a screen (or two) and a keyboard - is a matter of legacy, not optimization. The world's professional workforce is increasingly mobile, and another cycle or two of hardware flushing at major corporations will bear this out. As a coder you are one of an incredibly small number of people (relative to the whole) who really truly needs a "full fledged computer." Most others do not.
4) Add in the cost conscious blue collar world, the developing world, and youth of the world, all of whom are price sensitive (in general) in their own ways, and almost all of whom mot assuredly do *not* need a full fledged computer...and the trend is simply inexorable.
Tell you what. If you disagree, book mark this, and revisit it in 12, 24, and 36 months. The 4:1 ratio you cite will be reversed by 36 months. In fact, if anything, I'll take the Under.