The reason WP was so successful isn't because it's the best CMS, or even the one with the easiest API to learn for non programmers.
The reason WP was so successful is that, for a significant amount of time. it had VERY BASIC php requirements to run, but first and foremost, its API was very STABLE, allowing the creation of a gigantic plugin ecosystem that were still compatible after a decade, when JOOMLA, Drupal and co kept breaking API all the time, or decided to bump their minimum requirements way too often.
so there can't really be a wordpress killer, if you have specific need, use a more specialized CMS.
API stability is one of the most under-rated feature when it comes to open source projects.
The reason WP was so successful is that, for a significant amount of time. it had VERY BASIC php requirements to run, but first and foremost, its API was very STABLE, allowing the creation of a gigantic plugin ecosystem that were still compatible after a decade, when JOOMLA, Drupal and co kept breaking API all the time, or decided to bump their minimum requirements way too often.
so there can't really be a wordpress killer, if you have specific need, use a more specialized CMS.
API stability is one of the most under-rated feature when it comes to open source projects.