i agree that a ball that trickles into a sand trap is not going to have a plugged lie percentage but, its also not going to have a 90-95% lie either. ive learned how to hit sand shots over the years with the wide open club face but, its still not going to be as easy to hit as a light rough hit , percentage wise.
Heavy rough is heavy rough. if you're out playing and not cheating yourself by fluffing up a shot that lands in the heavy rough it can get pretty random. you may get a pretty decent lie,,, maybe. if you're lucky. but most times the ball drops down in the deep grass and getting a percentage that's anywhere near the light rough or fairway percentage just doesn't happen. in real life.
I don't know how anyone can expect HB to have a shot estimator that says, " oh its just off the edge of the light rough" so the penalty is much less then if its 10 yards off. that seems like a tall request besides, being just off the first cut and in the heavy rough can be just as bad sometimes. I agree that you have random lies when a ball is in the heavy rough. and, if someone creates a course that's got a lot of heavy rough and narrow fairways then that course is what it is. it may not be everyones cup of tea