That’s a very naive statement to make.
Absolutely majority of the companies will do just fine, because the hardware improves faster than their demands. Starting out with a distributed system "because one day we might need it" is just silly, because chances are you'll never hit it, and you'll have to pay for the overhead of having a distributed system (which is non trivial).
Actually my company started using PG and had presentation and someone asked if we considered a distributed database so we can scale. The presenter nicely said it was evaluated and this solution worked best, but that was too nice.
1. It's only about 100GB of data
2. The hardware is barely utilized, we didn't tune it (except some standard memory settings), because there's no need yet.
3. Our data is relational (in fact most data from most companies is relational)
Which means you can't use it for any big data/analytics use cases. MongoDB has fantastic client libraries e.g. Spark, Java.
This talk is a pretty nifty perf overview:
https://www.percona.com/live/e17/sessions/high-performance-j...
That said, if you know beforehand that horizontal scaling will be a crucial factor, probably postgres isn't the first choice. But with how fast CPUs are these days it's usually not important for a long time.