It would not be too hard for Microsoft to argue that they did come to an agreement with the artist to transfer the rights in exchange for 20,000 euros, so they

It would not be too hard for Microsoft to argue that they did come to an agreement with the artist to transfer the rights in exchange for 20,000 euros, so they do now own the copyright to the End Poem and will sue for damages if the artist reneges on the deal and starts claiming it's public domain. I hope they don't, but they definitely could. It would be a PR nightmare though, and there's not all much to be gained either (how much money can you make with the poem if you can't mention Minecraft?), so with any luck that will stop them.