I think the catamaran classes are starting to strike a nice balance where there is something for everyone. We have strong Hobie 16 fleets for people who want one-design racing on a simple platform. The A cat fleets are growing and present a great option for those who want a high-end development class (and can afford to pay for it). The Formula 18 presents a boat for those who want something in the middle of the road, and or a spinnaker boat. There are other boats out there (N20, H17, H20, I17R) but I think these three classes represent the core of catamaran racing in the US night now.
When we are talking about participation in the sport the biggest numbers will always tend towards boats that are affordable and easy to sail, that's the way things go in North America at least. The Hobie 16 is still the biggest catamaran class for this reason. We had between 500 and 600 class members racing Hobie 16s last year. By comparison the A class and the F-18 have under 100 class members last I checked. I can't remember the exact numbers but I did get them for a Sailing World article I wrote last year. All of these classes probably have more people racing than they have class members.
I think that if we continue the way things are going we will have some consolidation in catamaran racing and the result will be growth in class racing. By consolidation I mean less classes and more boats in each class. The Hobie 16 has managed to maintain its niche very well but I think the faster boats have had problems. The people who are drawn to these faster boats don't have class loyalty and tend to move to the next "cool" thing every few years (very uncool in my opinion). If speed is king class-racing takes a back seat. The Nacra 20s are starting to gain some class strength but will you see all these guys moving to Marstrom 20s next year? If it's all about speed they will. As a group I think catamaran sailors need to recognize that the speed culture is limiting the potential for class growth. If we can get past this and develop a range of strong classes I think we will see more mono sailors jumping the fence and getting catamarans and we will enjoy better racing.
This will NOT limit technological development because there will always be the tinkerers out there. You will never see complex, expensive cutting edge boats in large fleets but they will always be out there.