You would only lose messages if both people switched phones at the same time, and then probably only a single message. That tiny chance of losing a message does

You would only lose messages if both people switched phones at the same time, and then probably only a single message. That tiny chance of losing a message does not warrant allowing adversaries to read a message when the user has explicitley marked themself high risk. And even with the current implementation there is still the chance to lose messages.