Even though it is old, it seems like Canon would have made a better product than that.
As a rule, Canon’s L lenses are the best you can get in every way, including longevity, etc. And I know there are still lots of people who use the original 100-400, so not all of them are worn out. But on the other hand, as @nasa said there is a lot of sample variation, so it’s possible that the lens you got has an issue of some sort that it’s not supposed to have.
I did take a look at the pictures on Google Drive, and again, they all look pretty good to me! Of course, I’ve never owned an L lens, so I may not know what to look for in pictures taken with an L lens. Unfortunately, even those pictures were highly compressed, so it makes it very hard to judge “feather-splitting” sharpness.