Trying to convince everyone in the world to use the right solution for their actual problem the first time (and effectively going against Google's marketing machine, momentum of the general project, discounted plans, etc) is not my cup of tea.
IMO the right database for the overwhelming majority of startups is Postgres, vertically scaled until it needs read-replicas. The extension ecosystem, burgeoning support for configurable table access methods makes it a no-brainer. I don't have sufficient experience to back this statement (I haven't run that many startups, I don't run an accelerator/VC, etc) but I'd love it if some else that could refute it would show up.
1. Generous, hosted free tier. I don't have to fiddle with setting up a database or paying for it. And I really do mean generous. You can run a small business off the free tier if you code it in a way that doesn't use unnecessary reads and writes. I've done it.
2. Development speed. It's all schema-less, which is (sometimes) one less thing to worry about at the start of a project. Firestore is the fastest way to get a functional proof-of-concept I have ever seen.
3. Realtime. This means your frontend gets instant live updates out of the box. This can be pretty hard to achieve with other solutions, but comes free with Firestore.
4. Securely connect directly from the client. This means you don't need a backend at all, which is a huge win when you're just tinkering.
5. Amazing cross platform support. Setting up the connection to the database from iOS, Android, or Web takes a matter of minutes.
Now, a lot of these early wins come with growing pains later on. But early wins are still a great thing if it is the difference between your project getting off the ground, or failing from the start. I don't really use Firestore anymore, but there is a lot I liked about it.
