Minecraft should have a transfer ownership choice in a world. To prevent it being a half-realm, make it every 2-3 weeks until you can transfer the ownership to someone else. For example, lets say, I have a world. The world is unused by me and i don't like it. My friend can ask me for the ownership of the world and I can send him ownership and its basically a win-win.
If you really want it to be a transfer for some reason just delete the file after sending it to them.
(Of course that would rely on the honor system, there's no way to check if you have deleted all copies.)
Why would you want any limits on how many copies there are or how often you can send your copy to somebody else?
And why would Mojang/Microsoft care about enforcing ownership on worlds?
You can even use it as a half Realm if you want.
You can share the world by uploading it to a web site and you can download it and build something before uploading it again and then they can download it, build something of their own, check out your build, maybe modify it and upload it again so you can continue playing on it.