What good is a high MP count...

... when the majority of photos are viewed online at sub-1000px sizes?

I understand that it's useful if you want to print them, but let's just assume that only a tiny sliver of photographers actually do this.

What I'm curious about is how big of an improvement, say, a 20MP image resized to 1000px will look compared to a lower MP image, like a 4MP from an older Sigma.