I think you and the one that I responded to are confusing the abstract nature of a system with its practicality.
As much as Google would like to, they can't simply close their email servers for outside addresses and still call it an email provider. It would be a whole different beast.
The fact that you don't get to talk with other people only because an big email provider would disappear does not change the fact that email messaging systems are designed to be inter-operable. That is the key point. To try to argue otherwise is insane.