Olaf the Stout
Hero
The catch being, how many characters do you realistically use in a year? Do you make and play more than two characters in a single year? More than three?
Because, to use all the content found even in a small 32-page book you'd need to be making four or five PCs.
Even if you're playing Pathfinder Society twice every week with the same character on the normal xp path, you're only going to retire three PCs.
I think the flaw in your logic is thinking that every player will want to play every PC option in a book. Just because a book has 10 different character classes, doesn't mean the player has bought the book wanting to play every option.
There will likely be options the player never wants to play. Maybe the player doesn't like playing Wizards as managing all the spell choices is too much work for them. Or maybe he doesn't like Bards because he finds them too generalist and wants a more specialised character.
On top of that, even in classes that the player likes, there may be options that he doesn't want to play. The player could love playing Rogues, but doesn't like the idea of playing an evil character, so will never consider playing an Assassin.
Sometimes there will be options that just don't appeal to the player for various reasons. Once you pare all the options down, a book with 20 options may only have half a dozen that the player wants to try out. Now that may still be more options than one person could actually play before the next book with more options comes out, but it means the number of options isn't necessarily as it first appears to be.