Consumers buy the phones that are advertised to support features (e.g. LIDAR) and iMessage. They pay for the OS development.
Granted the SDKs would probably be much less comprehensive if there were no 3rd party apps but Apple advertises how their hardware can be used incl. by 3rd party apps and providing the libraries to access that hardware is expected by users (even if they don't think about this, most people expect Snapchat to access the camera).
Bandwidth/Hosting is covered under the $100/yr but if it wasn't, they could easily charge devs for tiers of downloads. Most of the support I'd bet is on the consumer side which wouldn't apply if there were other payment processors.
- SDK / iOS updates
- Bandwidth to upload the compiled app to their phone
- Support from Apple
- Hosting / security of aforementioned
- etc. etc.
Who pays for all of the infrastructure of the App Store and the Apple SDK then?