True, there's really no way to fairly compare movies from the early days of Hollywood to more recent ones. Inflation, ticket sales, nothing really works. Movies back then had many rereleases because there were no television showings or other methods. Maybe comparing actual number of ticket sales back then to everything including DVD sales. But even that doesn't work. Once someone buys the DVD. there's no way to track how many times they watch something or with how many other people.
I prefer to just say that "Gone with the Wind" is THE movie of Hollywood's Golden Age and nothing can ever topple it because nothing new can be fairly compared to it. It's sort of like the best hundred yard dash record will stand forever because they are not run anymore. They are now hundred meter dashes.