It’s not cheap, but on the other hand you get a lot. They handle the billing, credit card fees, returns, reviews, the site, bandwidth, updates, uptime (reliability ain’t cheap), security, and 10 other things I’m not thinking of. Oh, and the full updated toolchain and dev environments... Let’s also recognize that effectively the devs are paying some for the platform, just like the users are when they buy the phones. We all are paying in because it’s better (for us), otherwise go elsewhere... I would.
You as a dev just create a simple parked webpage for your app and you are done. So many barriers to entry are just dissolved. At no upfront cost to the developer. That’s huge.
I think the many many business that prefer to be on this environment, and always have, speaks to the value of that 30%.
Maybe 30% is a little high, it’s really hard and subjective to say. But it’s not crazy... there is a lot of value provided for that 30%.