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.