Greater resolution will obviously make more evident both flawed lenses and technique. The same is true of image size -- larger prints enlarge defects as well as the picture. But that does not negate the value of higher resolution. This is like arguing that all other factors being equal, Panatomic-x would not be more useful for large prints than Tri-X in the bygone era of black and white film. (Speaking of high reolution, does anyone here remember H&W control developer, which enabled Kodak Tech Pan and High Contrast Copy films to render a fairly full tonal range at incredible resolution?)
And yes, diffraction (scattering of light by the diaphragm blades) reduces sharpness, but if you need DOF, you must surrender diffraction sharpness to gain DOF.
Photography unfortunately is governed by the laws of physics -- and is not subject to what we want! It is kind of like computers, which also do what you tell them to do and not so much what you want them to do. (I must be telling this one to make typos -- it doesn't "want" to make me look illiterate!)
But to answer the question -- yes, I suspect the D800 is a great tool for landscapes. Look at Digital Darrel's post in this forum.