Yes, diffraction is happening in the lens regardless of the sensor, but the advantage of the higher MP sensor is lost much easier to diffraction. If you have two cameras, one 20MP and the other 282MP, and shoot both with a lens set with its aperture smaller than f11, there will be no difference in resolution between the two. The higher MP body won't be worse - it just won't have any advantage. But shoot at f5.6 with a great lens, and the 282MP sensor will be able to resolve 70MP of detail while the 20MP sensor is limited to 20. So while 282MP clearly holds an advantage with a great lens in that particular scenario, why bother with 282MP? Isn't the cut off of where a FF sensor should ever go to somewhere around 70MP or below? And if its a video optimised sensor, 39.3MP seems like the ultimate destination for FF if video remains based on a multiple of 1080p (4k or 8k), even hundreds of years from now.There are very sound arguments for up to 400mp, but with big caveats. http://theonlinephotographer.typepad.com/the_online_photographer/2009/02/why-80-megapixels-just-wont-be-enough.html
File sizes are a major concern when using these large sensors. As you said, the resolution of the D800 is too much, too soon for event shooters with current computers.
I was looking more into how far it is worth going based on the laws of physics, assuming sensor tech and storage/processing all continue improving, making these resolutions easy to capture and post process.
Going to the extremes of a 400MP FF sensor will allow you to see 400MP of beautiful blur up close when viewed at 100%, unless a lens exists to resolve 400MP of resolution at f2.3 and wider apertures. Having said that, software enhancements such as SmartDeblur, if further refined could turn those extra blurred pixels into meaningful detail:
I still reckon 20 odd MP is enough for most people, and 70MP without any artificial enhancement should be more than enough for any real life situation.
The article linked by privatebydesign indicates that raw engines overcome some of the diffraction limitations, otherwise surely a 7D sensor would have the same problem as the D800 and be noticeably impacted by diffraction after f/8?