One way I've seen this going awry is when real functionality starts being built as event listeners. Data that would ideally be propagated back up the call stack

One way I've seen this going awry is when real functionality starts being built as event listeners. Data that would ideally be propagated back up the call stack instead just get consumed via published events because then the core logic doesn't have to be touched. It's like setting up an observatory with a parabolic antenna pointed at the mysterious legacy core, recording & processing its emissions to reconstruct the inner workings.