Single machine postgres is mostly fine when you're working with less than hundreds of million of rows, less than a few hundred gigs of data, don't need HA, and don't need more than a few thousand QPS. Outside of those constraints is why new things exist imo.
Sure, small businesses are fine using postgres. I would argue that by developer market share though most (or perhaps just many) developers have to worry about these issues, since the companies that have scale have to higher many more developers.
There's also the argument that businesses hoping to scale might be better off assuming they will need it to save themselves work down the road.
This is an extremely important lesson. You don't have Big Data, unless you're one of a handful of huge companies.
Years ago I joined a mid-size public company and inherited one particular service. Management was adamant that it had to migrate away from MySQL onto Cassandra due to data volume. But there were no metrics being tracked, it was driven by "we have to do it because everyone is doing it".
I started tracking actual usage, growth and performance and it didn't seem to point to a problem that needed solving. So I halted that project (to their credit, they listened to engineering input so I was able to).
A few years later when I left, it was still perfectly fine on MySQL. Nowhere close to running out of capacity. Today it's been almost ten years and while I'm not there, what I hear is that it's still fine.
"But my data is more than 5TB!
Your life now sucks - you are stuck with Hadoop. You don't have many other choices (big servers with many hard drives might still be in play), and most of your other choices are considerably more expensive."
My point is that large companies that do need to scale hire many developers. FAANG likely hire north of several hundred thousand engineers alone, and according to this article[1] there are only something like 1.5 million software engineers in the US.
Even combined, FAANG is probably only around 150k programmers. A vast majority of the worlds 24-26M working software programmers do not work at them, or even at the top 100 such companies.
PostgreSQL handles 100k QPS without any issues unless your queries are very complex and I know many people who run multi-TB databases in PostgreSQL without any issues either. But it all depends on your workload of course.
