This is a very understandable concern, given the importance of having a platform on which you can rely.
Contractually Google Cloud provides a 1 year notice before discontinuing (or making backwards incompatible changes) to products. This is for generally available (GA) products. Cloud Run is in beta, so technically it could be decided not to bring it to GA. This is why some conservative orgs tend to wait for products to be GA before releasing them.
From a technical perspective, Cloud Run was designed to be highly portable and idiomatic. If the service were discontinued (or you just didn't like it), you should be able to take your container image, and run it anywhere else. Odds are you would be using some other Google Cloud Services, so you would likely want to run in an environment with low network latency to Google Cloud (Compute Engine and Kubernetes Engine being obvious candidates).
From a historical perspective, I'd say that Google Cloud goes above and beyond in supporting older products. App Engine is about to hit its 11th anniversary. We are still running PHP 5.5 apps and backporting security patches to the runtime, despite the language losing community support 3 years ago. We are still turning down an old product called "Managed Virtual Machines", which has now been in a deprecated (but running) state for longer than it was GA!
From an emotional perspective, I think that Google is eyed with a lot of suspicion for turning off products. Google Reader - enough said. But as someone on the thread pointed out, Google Cloud is a very different business from the rest of Google. Google (!cloud) is a consumer company at a scale where if a product matters when it hits a billion users. Google Cloud is an enterprise company. Scale still matters, but not in the same way it does in consumer.
I can't wait for hacker news folks to try Cloud Run. Its an awesome product.