The app dev put time and energy into understanding and optimizing for search within the store (and also keeping analytics), so to say they put 0% of their budget is not correct. Their time is money.

Other than Facebook and Google, what are the preferred marketing services for iOS indie developers in 2019? Is it mostly domain/niche specific, or are there horizontal promotion services for apps?

If Apple's 30% fee charged to developers does not include any marketing services, what additional percentage should be budgeted for the companies that are promoting apps? 10%? 20%?

Except that is part of the quid-pro-quo of letting them take 30% of your revenue, the idea that they will curate and market apps. Something they were doing for rooster8 and then inexplicably stopped. If we have to do our own marketing, then WTF is the point of the vig?

Apple quite literally is the market, and they refuse to allow external influences on the market's listings (mostly, sorta).

Apple chose to adopt it's existence as being a market and they're getting the revenue that goes along with assuming responsibilities which they are shirking.

Apple really needs to be hit with anti-trust investigations. Ditto for the play store.

Arguable some portion of the cut that Apple gets from app sales is to pay for the inherent "marketing" that the app store provides.

Except it literally provides this via the app store. Even paid targeted advertising.