In cases where the Wi-Fi is provided as a value-add or is bought via another channel than the Wi-Fi network itself, I think you can just generate one-time WPA Enterprise credentials, with a QR code to facilitate data entry?

In case of in-flight Wi-Fi, the credentials/QR code can be printed on the boarding pass, or available in the app (the app caches it in advance while it's still on the ground, so when in the air you can use those credentials to connect).

This doesn't cover 100% of use-cases but it would at least cover a big one (a significant amount of public Wi-Fi is "value add" to another service - whether restaurants, hotels, flights, etc where there's an existing channel to provide one-off wi-Fi credentials over), it's a shame nobody deploys this.