Oh, so it's a barrier-to-entry argument? That makes more sense. The barrier to doing something with postgres is pretty low, so competitors can more easily copy your ideas unless there are more barriers somewhere else.
An interesting point. More broadly: if what you're working on is not hard (and awkward), then others can copy your idea easily. Technology like postgres makes a new class of problems easy, and thus you need to find new problems to solve if you want a sustainable business.
pgSQL does what it does easily enough that it reduces the barriers to entry to such a level for traditional RDBMS workloads (which there are plenty of) that such workloads are simply not economically worth pursuing (especially for startups) except as small components in larger systems where the value add is elsewhere. The "other" world of big data/time series/graphs/nosql is hard to get right, and so is worth more, as if you crack it someone else copying you is decidedly non-trivial, meaning that it alone can form the core of a successful business.
This is a bit like what the web people are trying to do in mobile, where if HTML5 was magically the best cross platform mobile deployment option when we wake up tomorrow the value of mobile developers will collapse, and the web people will then cease to be remotely excited about mobile.