But could it be that the extra 30% Apple charges goes partly towards curating apps and ensuring they work well and are reasonably secure?

And that the high-spending consumer segment appreciates this?

Exactly this - I'm a "high spending" customer. Because they take pretty good care of their store, because there are no surprise auto-renewals and on and on I spend money freely - it's pretty safe to.

I always seem to regret signing up for subscriptions elswhere. Did anyone have the myheritage fight - I did that 5 years ago - total nightmare. I mean - how is stuff like THAT legal (hadn't used service in a year, no notice prior to the renewal being billed), no way to get out of paying.

We roughly know their profit margin on the App Store from corporate reporting though so seems unlikely to be any significant spending on curating, and a lot is to curate away competition like Steam's streaming that allowed PC game purchases to be remotely made through streaming your desktop and big picture mode.