You can also compile your own open source software from outside the Apple App Store, there are some limitations to long-term use that might require a $99/year developer account, but it’s still possible. And the $99/year is not per user but per company, so if you know somebody with a dev account, you can split it with them by starting a company instead.

Not to say that the iPad Pro is a Windows tablet replacement, to me they’re still very much distinct products right now. iPad OS isn’t (yet?) a macOS replacement for developers and power users.