That's not the only alternative. If iOS allowed sideloading or third-party app stores, banned apps would be available to iOS users in those countries. Since Apple is not directly offering those apps, iPhones would also remain available in those countries, just as Android device makers aren't banned for retaining Android's sideloading capabilities.

Or the app could just be offered as a web app.

That would be a fine option if it were fully supported by Safari. This excludes all web apps that require push notifications (including a hypothetical web app for Signal in China) until Safari implements the Push API on iOS. Safari would also need a web telephony API to answer calls for feature parity with the native Signal app.