To your point, Apple Developer does not require the $100/year Apple Developer Program fee to load software on your iPhone, but does by default require a weekly cert refresh.
However, the $100 is less a permission slip, and more buys you access to Apple services infrastructure that is largely worth the money so you don’t have to do it yourself and so your users can trust a single brand experience.
(For example, notifications before Apple’s notification service were insane, the app “Growl” made a lot of money just trying to tame the dozens of different ways confounded users’ expectations with notifications. Similarly, updating apps was crazy-making for users, then there was Sparkle, now you get the app hosting and distribution included in the $8/month.)
Just these few services are well worth $8 a month if you compare what you get to any other SaaS we’re buying all the time from HackerNews startups:
- App discovery, hosting, distribution, updates
- CloudKit, iCloud Documents, iCloud K/V Store
- Push Notifications
- Sign-in with Apple
- etc. (NFC is also in the list)
See this link for detailed differences between free Apple Developer and paid Apple Developer Program (also compares Enterprise distribution):
https://help.apple.com/developer-account/#/dev21218dfd6
Since you specifically mention push notifications which of course require an infrastructure to run reliably for you 24/7, there’s a good value for the $8/month. The systems behind making these “just work” for users are complex and expensive.