Earlier quoted context omitted.
Comparing one proprietary service to federated and open services? I'm not following you
While email as a whole is indeed federated, email service for a single company is usually not, that is you have a single provider and when that provider happens to have technical issues your email might be down. While obviously you could use other provider (or even free email) almost immediately to send emails, you will not be able to receive/access emails that are sent to your primary address/domain until the provid…
You won't be able to receive new e-mails, but if you use a local client like thunderbird, you can still access your old e-mails.
With slack down, I can only access the messages that I (luckily) have cached locally.