If the downtime persists for more than 72 hours some will switch as they need to conduct their business , whether its email/github etc. However as long as the outages are more like a snowday very few of the big orgs will have the inclination to get back into the business of self-hosting for accountability/price/biz inertia reasons.

A week long outage though , yeah at that point the decisions becomes a nobrainer and not even network effects will prevent the migration.

Even 72 hours isn’t enough - how will they conduct business with anyone else if they are still down and how will they get their old emails to resume conversations and how will they transfer their known email addresses or inform others of their new email addresses? None of that is cheap or easy.