Why don't companies with chronic outages mimic their stack from top to bottom (i.e. starting with a new domain), then before making a change, make the change on the duplicate stack and blast it with mock requests.

Might catch 90% of problems before they make it into the real stack?

E.g. every step of GitHub's migration to Azure could be mimicked on the duplicate stack before it's implemented on the primary stack. Is this just considered too much work? (I doubt cost would be the issue, because even if it costs millions, it would pay for itself in reduced reputational damage from outages).

EDIT: downvotes - why? - I think this is a good idea (I'd do it for my sites if outages were an issue).