A responsible Apple would also push these features editorially on the App Store. They'd write up articles about how they encourage responsible spending and have features to help you navigate the App Store and IAP responsibly.

Perhaps the default experience would include features like:

  1) The first time you spend more than $100 cumulative on IAP for Endless-IAP type items, a popup with a link to an article about available spending controls comes along with it.
  2) You get a similar reminder every additional time you cumulatively spend $100 on IAP for Endless-IAP type items (across the whole App Store). This could be disabled, of course.
  3) A more urgent reminder comes up at the $1,000 cumulative mark. It encourages setting an account-level monthly budget. It takes more work to disable.