Tha is true. I do not have the list but there are some countries in that ratio.
I think that they can create other methods to increase the paybase (i created a thread about it, i should have included the poll because i didn't know about its existence).
Inside the game, paying at discount (3 times per year) girls, 10 epic pachinco or something like this.
Outside the game, lihe suggestions, it could be possible some kind of merchandising (perhaps) that you can produce according to the demand. You can think about pens, notebooks, cups, etc.
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In fact it could be an option dropping "standard girl" from super pachinko with a super super low rate xD. But perhaps for one or two girls.
Like an idea they could include here some past girls that you cannot obtain. One or two.