A common usecase for WorkerRunning listeners seems to be that they perform logic based on $event->isWorkerIdle(). E.g. sometimes services only need to reset if a message has been processed. Or non-critical logic needs to happen when the worker is not busy, to keep the processing performant (like keeping a cache warm or sending buffered metrics).
Right now each of those listeners need to be called, upon which they check themselves if the worker is busy. To increase performance, the WorkerRunning event can be split into WorkerBusyEvent and WorkerIdleEvent. This way listeners can be explicit about when they want to act. Since these events are emitted in a loop, this could make a difference.

There doesn't seem to be a way to deprecate an event, so I think the WorkerRunning event remains.
Though WorkerRunning::isWorkerIdle() could be deprecated to stimulate the usage of the new events.