(my humble opinion:) It all depends on the amount of friction this adds.
If you would make this really smooth transition in Amethyst (e.g. show a popup that the app will be paid soon and link to a website that allows you to download the free option apk and then somehow make it easier to transfer private keys), then I'd guess like 30% of people would actually do it.
If this just happens one days and there is no discoverability of the alternative, then I'd say less than 10% of people would get the free version, others would get Plebstr, Iris or just leave and complain about it on Twitter.
The number of people dropped from Nostr would be the interesting metric to watch. My guess would be 50% of current Amethyst users would drop completely from Nostr.