Your data is your most valuable asset, and by using this you're locking it inside Google servers. If they decide five years from now to discontinue it, or to raise the pricing 10x, you're screwed.
Are most developers only working on short term projects? Why would you put yourself in such a situation instead of using open source technologies that can be deployed anywhere?
You'd be able to move your data off of firestore. And there's legal business contracts around pricing. Google can't just raise pricing 10x overnight.
It's a very expensive move that I don't think people consider when choosing this kind of solution.
If you're concerned that a service might shut down then you need to architect your application with that in mind, in which case how much of a rewrite is necessary is essentially up to you. Usually there's a tradeoff between going fast and engineering solutions that will work in the long term. Most startups never get to the stage where they need to swap out a service, so closely tying your application to a service is probably OK at the start.
If the service that shuts down is reasonably popular though it's likely there'll be very little code to change. API-compatible competitors will pop up to replace it. It happened when Parse closed.
[1] https://appbase.io [2] https://store.docker.com/images/appbaseio [3] https://appbaseio-confidential.github.io/streams
It seems a lot of developers use the HN stack: eg things thatvwent thru YC or that they hear other devs talking about a lot on HN.