IIRC they've had periods when they've let it be a free for all and I don't think it worked out that well. Obviously having less penalties around the ruck would make a better product, but diluting rucks wouldn't make it a better sport. I'm lucky being both a league and rugby fan that I get to enjoy a sport both with and without rucks.
TBH, more penalties in rugby v league doesn't worry me as the nature of the sport means that there's more opportunities to give them away, it is what it is. Some are annoying, but then the whole hip drop tackle business in the NRL this season has confused the fudge out of me.