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Dave,
Diffraction is real, but focus stacking can be a very effective tool for landscapes as well as macro. Or for the Canonites, just tilting your 17mm TS-E can be a pretty effective way of getting wide DOF w/o objectionable diffractive blur as well.
Also, while diffraction does start to become more evident above f/11 or so on the 7D this isn't a binary effect - it comes on slowly. As a result significantly more detail will still be resolved as pixel density increases beyond this - even at f/11-16. It just won't improve as much as was the case at wider apertures. We won't reach the real point of diminishing returns due to diffraction until over 50MP on APS-C or well over 100MP on FF (though noise issues may become objectionable before then).
Finally the D800 at 36MP on FF has less pixel density than the 7D, so while I agree that there's no free lunch, I believe the D800 still offers an appetizing meal for the landscape photographer. (But I ain't trading in my 5D2).
PS. To strain the free lunch metaphor even further: There may be no free lunch in optics and physics, but optical physics has payed for all my lunches (not to mention cars 'n houses 'n stuff) for the last 30 years.
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