I don't believe you're accurate. All apps that could charge money through Apple payments stopped doing so long ago and now are "free" with an external subscription or ads: Netflix, Amazon Prime, Microsoft Office, Slack, Google Docs/Drive, Dropbox, Facebook, Instagram, TikTok, Hey Email, etc. Only free-to-play games / gambling / scam apps stayed in the App Store, and the rest are gone.

It’s also worth noting that the “developers” who are able to escape this all need Apple’s permission and Apple almost exclusively grants permission to megacorps that could single-handedly hurt Apple’s business were they to protest. Smaller (which in this case doesn’t necessarily mean “small”) players are still locked in. This has the effect of further entrenching Big Tech since they can escape the confiscatory Apple tax while smaller competitors cannot.

> This is empirically not true. If the value-chain is so 'ruined' and 'uneconomical'. Why are there so many iOS devs?

Survivorship bias. You only see businesses that have high enough margins to eat the fees. Whether that's a desirable set of incentives for long-term prosperity of everyone involved is a different question.

Apple could raise their fee to 90% and there would still be plenty of iOS developers using in-app purchases, because selling a few bytes in a database is effectively free. It would all be trash, but they would definitely be there.