I say we keep the Slapstick Effect...at least at the beginning of the Fenspace stories. Perhaps the effects of handwavium are changing as it interacts with humanity? We already know that handwavium can mutate into different strains, after all. The slow loss of the Effect might be due to that....or it might actually be a programmed event - a delayed reaction to allow the goo - or humanity - to spread far enough apart before it allows full weapons effects?