I might be adding to a pile on (I wouldn't say this is a pile on, it's more of a theme of the day today) but I feel like I could add my two cents to the discussion, since apparently my History of Cinema class was not completely useless after all!
When the film industry gained momentum in the 1920s and 1930s, it was a completely different industry than what we see today, there were a handful of major studios and these were the ONLY studios making movies. These studios didn't only make the films, but they also held exclusive contracts with actors, directors, writers, etc. Anyone who made a film for Paramount in the 30s, 40s, and 50s was pretty much owned by Paramount. If you look at the credits for movies made before the fall of the studio system, you'll see dozens of writers and editors. If you look at a famous actor/ress's career prior to 1960, you might see an overwhelming amount of credits. Content was constantly being made. The studio executive had complete control over the finished product. There was no independent market.
Essentially, during this time, studios would just crank out tons and tons of films. They sold films to movie theaters in blocks, which meant that out of 5 films a theater was showing from their studio, maybe one of them was one that people actually wanted to see. If a person went to the movie theater, they might pay a nickel to see an entire day's worth of films. On top of that, by the 1940s, there were more movie theaters than banks, which I guess isn't saying much since it was The Depression, but still, movies were everywhere, they were always playing, and they were readily available, and they were relatively cheap. People sat in movie theaters all day back then in the same way people sit around and watch Netflix all day today.
Because of that, comparing the film industry of yesteryear to the industry that it has become today is just impossible. If you want to compare the Hollywood Golden Age of Film to the entertainment industry today, you should look at the television industry and you'll find a more comparable system.
In fact, most of the golden age studios own the major television companies. Not only that, but major broadcasting companies own a significant amount of the obscure channels out there. For example, Disney owns ESPN and ABC and subsequently NBC sort of, since ABC and NBC are forever entwined somehow. CBS owns channels like TBS and TNT. And Viacom and Comcast own so many random things I can't even keep track of. Everything trickles down to these big conglomerate entertainment industry companies and the money you spend on a DVD box set of Lost eventually goes into the pockets of Disney executives. Instead of trying to compete with the independent film market, they just found a new market to dominate.
After 1960, movie making completely changed, even in this decade, the industry is still changing as the filmmaking process becomes easier and equipment becomes more available to the average person.
When you talk about the success of a film like Gone with the Wind or Casablanca, it would be better suited to talk about it in a similar way that one might talk about the success of a television show like Friends or M*A*S*H, than comparing it to Frozen or even the early Star Wars trilogy. Movies back then were just not the same, and using them in comparison to the success of a modern film is like comparing apples to pears; a relatively similar fruit, but significantly different.
If you are comparing the highest grossing films in the past 4-6 decades, then you will most definitely have to adjust for inflation to get a better idea of how successful a theatrical release was in comparison to an equally successful past theatrical release. Like I've mentioned, about 10 years ago I was paying a mere $8 for a movie instead of the $11 to sometimes $20 tickets today. I just don't think films made prior to 1960 were made and released in the same way that modern films are and using them in comparisons is pointless, and honestly, does not negate the success of either film.
The truth is, whenever a film markets itself as the "highest grossing" or "number one" or "greatest film of all time" they are just lying. That's it. For some reason words like that make a film more appealing and it is just a marketing tactic designed by a lazy advertising department.
Also, Citizen Kane is overrated. Shots fired.