Those are very interesting numbers, but per usual you dramatically overstate your case when you say admissions are the "correct" way. Like any metric, it fails to account for a lot. For example, you yourself said that there were fewer entertainment options in 1939 and that people went to the movies more often. But this makes Gone With the Wind's totals more impressive, not less:
No, it makes less impressive because it is easier to get X admissions when people go more often to the movies. It is easier to get 200 million admissions if you have a market where there are two billion admissions per year than where you have one billion admissions per year.
That Star Wars managed to reach 90% of the number of admissions of Gone with the Wind when the total number of people who came to the theaters was smaller is much more impressive.
the more moviegoing there is, the lower each film's relative percentage of admissions will be.
Not if the number of movies being released is comparable. In 1939 and 1977 the number of movies released in US theaters was rather similar at 600-700 movies released per year.
It's easier to dominate that measure when fewer people are participating.
No it is not. The number of admissions per year in a country don't have any direct effect on the difficulty in obtaining a percentage of total admissions to your film. In other words, if there are 500 films released when there are 1 billion tickets sold and 500 film released when there are 2 billion tickets sold, therefore the first case would be much harder to attain the same number of admissions as the second.
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Gone With the Wind is also nearly twice as long as Star Wars, which means fewer showings per day, which cuts heavily into admissions. And so on.
Perhaps. Cinemas would have less incentive to show Gone with the Wind because it was much longer.
There is no correct way, but if I had to nominate one, it would be inflation-adjusted gross, because the price of the ticket is fluid and therefore takes a lot of these other things into account for us. If people are going to the movies far more often, prices are lower, which is reflected in the gross.
Inflation adjusted gross is the number of admissions.
It is much easier to make a film gross 500 million in 1939 than it was in 1977, in inflation adjusted dollars, because the market was bigger. It is much harder to make a movie gross 200 million in Japan than in the US, because Japan is a smaller market. A movie needs to be much more popular if it grosses 200 million in Japan which is roughly equivalent to grossing 900 million in the US.