> This was the very first time I heard anyone even suggest that storing data in Postgres was a concern in terms of reliability

You seem to be reading "reliability" as "durability", when I believe the parent post meant "availability" in this context

> Do you actually have any concrete scenario in mind? Because anyone can make any system "considerably degrading", even Redis

And even Postgres. It can also happen due to seemingly random events like unusual load or network issues. What do you find outlandish about the scenario of a database server being unavailable/degraded and the cache service not being?