Something that puzzles me: open license, yet so arcane that it usually must be consumed as a service that costs far more than closed DB licenses usually did.

At some point complexity and lack of polish becomes indistinguishable from closed source. An open system I can’t use easily myself is not open even if the source is available.

But in those days, in addition to the license prices for closed source DBs, you also had to pay all the hardware costs to run them on, and hire an army of DBAs to support and maintain them for you.

Running HA Postgres is not easy and requires a lot of niche/specialized knowledge. But running one of the big commercial DBs back in the pre-cloud days required just as much specialized knowledge at the DB level. It required network engineers and Sysadmins. It was far from cheap to run at scale.