I can't imagine any landscape photographer would want a 46.1Mp tool with diffraction limit starting at f/5.6 already.
"starting at" is the key... AFAIK other than very large prints it may not really start to be visible until much smaller apertures. Neuro or Jrista can tell us more I'm sure. But fair point, for serious landscape medium format would still be leaps and bounds ahead.
Here to save the day!! ;P
Actually, diffraction starts at f/6.9 (same as 7D, which actually has the same pixel pitch as a 47.6mp sensor...so a 46.1mp sensor would probably actually have a DLA of f/7.1 or so). And the diffraction limit is simply the point at which the airy pattern (vs. the airy disc, which is the central peak) STARTS to affect surrounding pixels. The outer region of an airy pattern is generally a far lower intensity than the central airy disc. You can keep stopping down beyond the DLA, as it would be a couple stops before the airy disc becomes large enough to affect more than a single pixel and exhibits as detail-eating diffraction softening
You also have to keep in mind...a sensor with smaller pixels will only experience diminishing returns
beyond the DLA. Once diffraction starts affecting resolution, that does not mean that your awesome 46.1mp sensor will have WORSE quality than a lower-resolution sensor. Simple fact is a lower-resolution sensor is physically incapable of resolving as much detail as a higher resolution sensor...always. In the absolute worst case scenario, say f/22 or f/32...or f/64, a 46.1mp sensor will only ever produce images "as bad" as a lower resolution sensor. A higher resolution sensor will never produce images that are "worse" than a lower resolution sensor (from a resolution standpoint...noise is a different aspect of IQ, and out of context here.)
Until you reach the diffraction cutoff frequency (the point at which pixel size is the same size or smaller than the wavelengths of light), you can continue extracting more detail with higher resolution sensors. As such, a 46.1mp sensor will always be better than a lower resolution sensor, regardless of the aperture used. Even at f/8, f/11, f/16 for landscapes...a 46.1mp sensor is going to keep extracting more detail...even if its minimally more, its still going to be more than say a 36mp sensor, a 22mp sensor, an 18mp sensor.