App Engine is indeed problematic. I have an important app on it and various forced upgrades gave me a big headache. It's been stable for several years now, so I'm okay with it, but as much as I like the idea, and indeed the execution, of App Engine I'm not going to do anything new on it because of the lock-in factors.

Cloud Run, however, uses standard containers, so as long as you don't use Google proprietary stuff on the backend it's relatively easy to move. As the article mentions, it's useful for low-traffic projects, and if they pick up you can move them to full-time instances.