It isn't just a simple case of logging into a different character while you wait. Consider the fact you cannot cancel the transfer half way through, this means something is happening at the back end that could cause issues if it was stopped halfway through. With this in mind, the Devs are still going to put plans in place for when it DOES happen. This is going to require the server to find your character, make sure nothing was broken (likely by comparing it to a backup) and try and get it back to the home server. All of this takes time. Now, they might also have tested what happens if a character is in this state and you try logging into another character, who knows what could happen to the one stuck in limbo, waiting to get back as you have effectively issued a log out command to a character that isn't even logged in.

Now, I'm not going to claim to be an expert by any stretch of the imagination, but blindly saying that no testing was done, when the complete opposite is most likely true, shows some level of ignorance on the subject.