The only thing I have heard of wearing out, is the black coating on the lens, but we are talking a lot of use to to get to this point. An example is a photographer who shoots lots of MotoX, with dirt & grit everywhere. I would expect the normal amateur to get nowhere near that point, especially with a macro or portrait lens.

The Nikon lenses are built better & will take more abuse, but as you said, they cost a fair bit more

Hope that helps, Dave