As evidence for this, I note that there are very few people whose "known for" list on Wikipedia is nearly as long as von Neumann's, and you'd expect more such people if the productivity difference between the 1st and the 10000th weren't very large.

http://en.wikipedia.org/wiki/List_of_things_named_after_Leonhard_Euler

"Mathematical historian Eric Temple Bell estimated that, had Gauss published all of his discoveries in a timely manner, he would have advanced mathematics by fifty years"; http://en.wikipedia.org/wiki/List_of_things_named_after_Carl_Friedrich_Gauss

http://en.wikipedia.org/wiki/Category:Lists_of_things_named_after_mathematicians

(This isn't to contradict your point, just provide relevant evidence.)