You can sell the phones alright, and they might even work, but the fact is that participation in society - especially if you live in a city - will be much harder without Android/iOS.

Note, not impossible: You can always carry cash to avoid phone-based bank payments (which would be needed at e.g. my local farmer's market, where nobody has a card payment terminal), some taxi services (Yandex Go for example) provide a web view with some of the features, you can open map services in the browser ...

But for the browser-based cases the experience will be even worse than the standard app experience, and friction is overall much higher.

As a result, only a very small fraction of nerds are committed enough to buy and use these devices. You then have a chicken&egg problem about getting a third option to work.

The only way this has been done semi-successfully in recent years is Huawei's HarmonyOS - and they did it by way of a) already being an absolutely massive phone company, and b) keeping around an expensive Android-compatibility core for many years.

Sailfish OS isn't fully FOSS. I believe the UI is still proprietary.