I'm biased having worked on GCP, but I think GCP actually has a very good track record of not sunsetting entire products or removing core functionality. When I worked on AppEngine, I would often see apps written 10+ years ago still chugging along.

It is true though that GCP sometimes sunsets specific product functionality, requiring changes on the customers' part. Some of these are unavoidable (eg committing to apply security patches to Python 2.7 given that the rest of the world is mostly not upstreaming these patches anymore), but not all of them.

I would certainly use Cloud Spanner externally now that I've left the company, and IMO spanner has such a compelling featureset that it's a strong reason to use GCP for greenfield development. The problem though is that it's only available on GCP, and could become expensive at large scale.