Hi Mary,
I would say the decision to change the shape of the race course from the triangle to windward leeward has had the largest effect on beach cats. It opened the door for the unirig with spinnaker sailplan to be competitive and with this opportunity came a new family of boats, F18 classes etc. The adaptation and development of the spinnaker to the beach cat led to the decision to change the shape of the race course.
The race course shape change could have been to go to a square or rectangular geometry with approximately equal time or distance spent on each of the three basic points of sail. This would have left the high speed jib reach in the race and the unirig/spinnaker rig boat designs would likely have never happened.
Bill