For anyone wondering "how bad could it be?", consider not only the ESO example, but that this was supposed to be implemented in March. But because of a confirmed "bug," it got delayed indefinitely.
AKA I'm betting the process is so bad and unreliable that it would have been another "4 months down for steam users" like ESO and SE didn't want that happening before ShB launch.
I fully support a petition for SE to demand Valve let Steam users switch should this change lock Steam users out of the game for any significant amount of time.