Having an outage, fixing it, and then not having outages in the future, may be a positive signal compared to no outage ever, because you assume it will happen and the recovery time and process is unknown.
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.
So again rationally this shouldn't affect the stock price much.
However repeated disruptive outages every few weeks/months (à la GitHub) is a different story, and there's also the invisible effect of businesses maybe being less likely to choose Microsoft in the future