It really depends on how much time people would have to migrate data and swap email accounts. If it’s years, we would probably be fine although the financial and data loss would be still be incalculable.

If we didn’t have much or any time to migrate… I don’t even want to think about it. If email accounts suddenly got shut down many people would lose banking access, among many other things. It would probably cause a global recession if I had to guess (talking out my ass on this one but I wouldn’t be surprised).