Data has more value when combined, so a wise database choice also depends on what you already have. You can combine data from different systems (e.g. FDWs, which postgres has great support for), but something is usually lost along the way.
I think postgres is a good default, because it is good for a lot of things. But sure, if you make an informed decision otherwise, there's nothing wrong with that.
It doesn't matter whether you have your needs clearly defined, Postgresql is a jack of all trades and a master of many.
An abstracted interface for your datalayer is a must have.
Many startups and projects begin with a single db and grow into new dbs as the business requirements change.
Obviously if you know your data model well enough you can foresee a lot of these requirements and pick the right tool for the job.
However, you will usually need to pick a db to start with and hope that it will accommodate as many of those unknowns as possible.
I believe that Postgres is the best choice in that scenario, as it is extremely mature and has an incredible amount of flexibility.
Somehow that makes me feel better that at least, I'm on the right path.
The reason why Postgres is so popular here is because if you want to have an open source solution, Postgres is the best available. I'm sure there are better proprietary solutions, but even though it is free, PG is pretty darn close to them.
For instance, at my current startup, we employ at least 7 different databases (including PG). And, I don't say that to brag - each has a specific use for the problem at hand.
You have to consider the needs and trade offs of your specific project. And, if you can, try to isolate your storage behind some interface. Because often your needs will change.
(I say this with 20+ years of experience and over a dozen commercially-successful products in my belt)