I would love to see a libre version or a version not released on Google Play. It would be possible to monetize it like FairMail or Threema Libre. It would be a win for all parties.
Does it? The purchase can be handled in-app by using for example PayPal or any other money processor. And then you can distribute the app however you want with the added benefit that folks without Play Services (like me) can enjoy it as well. I had to switch from Sync to Relay when I switched to a phone without Play Services.
Distributing the app outside of a store does have security implications though from the app developer’s perspective.
They have to implement a key system and a way of checking it. You can’t just allow people to download the app if you want to charge for it as they’ll be able to give it to friends for free.