Do you know of any known public example of someone who has actually done that? This seems advise similar to eating 6 portions vegetables a day: everyone says it's critical, nobody is doing it.

In all the cases of Amazon and Google Cloud outages I've seen that were beyond a single zone, it all hit the high-valued companies hard. I've never seen anyone declare that they decided to activate their alternative provider where they have a continuously updated copy of all the data so the data loss is minimal.

Everyone is just assuming Amazon or Google will fix it and that any data loss from activating a non-hot standby is not worth it.