Why does everyone run ancient Postgres versions?

I say this as someone who absolutely loves using it, but the actual process of upgrading Postgres is something that takes significant downtime, is error-prone and you're often better off just dumping all the databases and reimporting them in a new folder. (A good idea in general since it'll vacuum and compact indexes as well if you do it, combining a few maintenance routines in your upgrade path.)

It requires having the previous version of Postgres installed, something which can mess with a number of distro policies (not to mention docker, which is the most popular way to deploy software that will typically rely on Postgres), and unlike most software with that issue, Postgres is software you want to be managed by your distro.

Therefore, most people only upgrade by necessity - when their distro forces the upgrade or the version they're using reaches EOL.

1. Leave your old database running

2. Start your new database with new version

3. Run logical replication and let it catch up

4. Decide you want to switch over.

5. Using a script: Stop your apps writing to the database (or switch to read only mode), let logical replication catch up one final time, stop logical replication, run a procedure to fix your sequences, and switch your apps over to the new database.

You are now done. Sure, it requires a one-time investment to work out the exact steps needed for your situation, but it's easy to do with minimal downtime.

Of course if you don't care about downtime just shut down your database server, make a backup(!), and then do pg_upgrade.