What is enough MP?

I must admit to being a little confused by the arguments over MPs.
The 5D3 is 22MP, giving 3840 lines in the picture height.

The very best lenses seem to have a resolution of about 3500 lines per picture height (in the centre, ignoring the edges).
Doesn't that make going for much higher MP a bit pointless?
Seems to me that if you want more resolution you need to go medium format.

Or am I missing something?