>> I can’t help but imagine whether Google Cloud’s low market share is related to this. If the internal culture is that clients must act multiple times a month to deal with impending breaking migrations, at least some of that attitude likely falls on external customers.

We've been GCP customers for five years and I can't recall offhand an instance of a forced migration due to a breaking change. They give a lot of lead time and plenty of notices, and then continue to support the old thing for a good period of time. It makes sense to me that they would treat outward facing APIs differently given that they span many organizations by definition.