I also want to add that in addition to being taller AND throwing harder, that command has gotten better over time, not worse, especially by the hard throwers.
The two kings of fastballers, Feller and Ryan, averaged 4.1 and 4.7 walks per nine innings, which is not good. They were marvels in their time and they did possess velocity as good as the best today, but they were rare in that regard...and their control was not as good as the flame throwers of now.
They were freaks of nature in their time due to their velocity, and now they would just be another pitcher in the clubhouse in regard to velocity, but with the organization waiting for them to develop command before they were good enough to contend with the big boys for awards and the big contracts.
In all of this, I do not want to minimize Koufax though. Koufax did have a great prime good enough to be in the discussion, even considering everything I've said, and it is unfortunate we didn't get to see how he would have held up. Just that Johnson was better
.
PS, and a nod to Ryan for reinventing himself to gain command and that knowledge did help pitchers like Randy Johnson. The early players get their due to being pioneers, as without them the next generations of players wouldn't have learned more by watching the best of the previous generations.