If that person does not own coins on any existing blockchain, he can't send any coins...
Right, but it's not our case.
Then excuse me, maybe I did not understood your post. Maybe you meant coins existing on a certain address on only one blockchain/a transaction only mined on one chain? Either way, one blockchain will be dropped... But anyways, my opinion is just that, my opinion, not based on facts. Just an hypothesis, like your first post. Unfortunately crystal balls don't work, at least yet
