Dropping development of prooph/service-bus has other reasons. The component is designed to run in a modular application - think of ZF2 MVC or Symfony bundles. Since 2014 things have changed. The Zend Framework team promotes usage of zend expressive. Symfony made it easier to develop bundle-less applications. That said, nowadays both major frameworks advocate smaller application size with less layers and less complexity.
More info: https://proophsoftware.github.io/event-machine/news/2018-09-08-future.html