Best landscapes for example I have on my walls are made with 5D classic which is 12mpx I believe.
I am using L's and I don't print larger than 24 inch, on this size all details are excellent. I don't need to see pimples on girls face 5 miles away shooting landscapes with wide lens, just no need in this I shot airshow 3 years ago with 1D3 and all 24 inch prints are excellent too, 8mpx? How do we all survive before gigantic mpx of D800? Nobody printed large before?
Typically in the film days, people shot with either medium format or better yet large format if they wanted to make large prints. The same premise exists in digital...the more pixels, the larger you can print and still contain the fine details. Again, good lens and good techniques are required or you are just wasting pixels.
Many people make large prints and claim the details are fine...but their fire fine details in the image. From air show photos I have seen, they typically don't possess fine details like a rain forest image...so you can print it large and it will look fine. However, images containing minute details shot with a low mpix camera will start to deteriorate very quickly to mush when printed too large.
I guess we all have our own standards dictated by what we shoot, how large we print and just our own view of what is good enough.