As far as my memory serves me right, OS 3.5/9 ReAction sources were not available to developers of OS4, so mot work was done from scratch or based on older ClassAction.
Even more interesting is the fact that newer ReAction classes were available in 68k forms and work fine on the classic systems, so backporting is not a BIG question, if it was a goal, it could be done fairly quickly imho. Sadly it was not before, maybe that's exactly what it is happening now. But definitely, reinventing things every few years is a big disappointment and humbles progress. But I would support it anyway, it is still progress and we need it.