Some of those guys may have taken their techniques to the grave.
As a make-up effects enthusiast, I can confirm that some did. There were some cases of understudies, but a lot of the early masters took their secrets to the grave.
Your line of thinking is probably the most accurate, but wouldn’t the change in film resolution, as well as any number of tiny variables like lighting and lenses also affect the ability to pass off make-up as reality? I am sure our HD generation is part of the issue when it comes to believable makeup.