I don't think there's anything unscalable about Postgres, or RDBMS's in general. I've seen even poorly tuned Postgres with unnormalised table designs work fine at a decent scale, to the point where I'm convinced that Postgres with a decent table design gets you very far.
As in: far enough that if you outscaled it, you'd be able to afford a team of excellent engineers to write an appropriate database system.
Almost all companies don't need the hyper scaling NoSQL databases supposedly promise. What they do often eventually realise is that they want the querying power and additional ACID guarantees of a typical relational database, so they end up developing a shitty relational database on top of a NoSQL database.
As in: far enough that if you outscaled it, you'd be able to afford a team of excellent engineers to write an appropriate database system.
Almost all companies don't need the hyper scaling NoSQL databases supposedly promise. What they do often eventually realise is that they want the querying power and additional ACID guarantees of a typical relational database, so they end up developing a shitty relational database on top of a NoSQL database.