GitHub CI lets me test my package, for free, in all these: https://github.com/ncruces/go-sqlite3/wiki/Support-matrix

Which is actually useful.

> just have some sane rules and make everyone follow them (like run unit tests before submitting a PR)

and thus you discover the value of CI

The big deal is that GitHub provides it for free. Plus it integrated properly into the PR workflow.

Good luck implementing merge queues yourself. As far as I know there are no maintained open source implementations of merge queues. It's definitely not as trivial as you claim.