PHP also cares a great deal about backwards compatibility. There are long deprecation phases before anything gets removed.
The RFC system also helps a lot in ensuring that changes to the language are well thought out to and discussed beforehand. The is not much change just for change's sake. Churn is kept to a minimal while still allowing the language to evolve.
> There must be a lot of unmaintained PHP codebases that will break if PHP is updated by hosting provider etc. Someone must be pulling a lot of hairs because of this.
PHP upgrades are also good money for agencies. Got to earn that bread.
And to let out a dirty secret, for really hopeless projects there is the option to keep them on life-support by running them inside a docker container with an older PHP version. (Horrible but if that's what the customer wants, it's their risk.)