Why does everyone run ancient Postgres versions?

Upgrades are hard. There was no replication in the before times. The original block-level replication didn't work among different major versions. Slony was a painful workaround based on triggers that amplified writes.

Newer PostgreSQL versions are better. Yet still not quite as robust or easy as MySQL.

At a certain scale even MySQL upgrades can be painful. At least when you cannot spare more than a few minutes of downtime.

"Not as robust as MySQL"? Surely you're joking.

They’re not wrong. If you’ve ever spent meaningful time administering both, you’ll know that Postgres takes far more hands-on work to keep it going.

To be clear, I like both. Postgres has a lot more features, and is far more extensible. But there’s no getting around the fact that its MVCC implementation means that at scale, you have to worry about things that simply do not exist for MySQL: vacuuming, txid wraparound, etc.

My experience has been exactly opposite. Ability to do Vacuums is good. MySQL doesn’t free up space taken by deleted rows. The only option to free up the space is to mysqldump the db and load it again. Not practical in most of the situations.