You look at the history of attendance at some of the bigger races like Spring Fever or Tradewinds published in some of the other threads and you see an increase in some of the specific fleets. There used to be big numbers though in the open fleets and these have almost entirely gone away, so the overall attendance is way down.

I will not argue the advantages of sailing 1 design. However with current attendance and the sheer number of designs available, having more than 1-2 events a year with a large fleet of 1 design is not really feasible. Traveling more than the distance to my local lake to race just a couple of boats is not very appealing so no wonder there is not much attendance at races. Nobody really wants to race a wave against a carbon 20, but the bigger attended fleets, F18, A class and F16 all rate extremely close. A start with 20+ boats, now is something that would be more worthy of some travel.
Segregation of the classes is not the cause of poor attendance, but I do not see it helping either.

From a training point it is a huge problem with US sailors trying to compete internationally also as we almost never get a big fleet start. If you can’t practice this, getting off a line in a 70 boat fleet at World Cup event is going to be problematic and pulling a miracle from the back of the fleet just can’t be relied upon if you expect to compete for a podium place.