You say potato, I say potato. To someone who spends their working life in gmail (or whatever) having it suddenly go away, being replaced with a different tool which requires different workflows is certainly a "disaster" in some sense. It's an equivalent annoyance, for example, to losing one's laptop or phone and having to restore a new one from scratch (assuming you have working backups anyway).

What it's not is incontrovertible proof that the new one is "bad". It might be great, but it's still a net loss of productivity while the users learn it.