There's a nice facebook study on how frequently people were not receiving cached images, and it was around 44% of all users (25% of all hits). If you figure that Facebook is going to have some of the better numbers for that metric, it supports your conclusion: https://code.facebook.com/posts/964122680272229/web-performa....

There was a similar exercise done with hosted versions of jquery, but I can't remember who did it or find a link, I'm afraid.