If you look at the world of indie tabletop RPGs, for example: Kevin Crawford of Sine Nomine Press makes a very good living and a significant percentage of it is "passive" sales of his back catalog. But if he stopped publishing and promoting new game projects, sales of that back catalog would very likely shrivel to nothing within a calendar year.
The open-secret ingredient is always more work.
It's why someone like Crawford can afford to tell everyone exactly how he does what he does... Giving away extensive production files that show you his whole creative process, soup to nuts: 99% of people aren't going to put in the work necessary to sustain the passive portion of an individual income.