I remember IDEA did a survey on this topic in the last year. this is a link to some of their data:
I think Harris is right because trends can change so quickly. One can purchase a lot of equipment, pay for expensive training, and then whoops, people have moved on.
I have seen a huge growth in my own specialty, yoga. Given how fast yoga teachers are being produced by training programs you would think we would hit saturation, but each year it does seem to continue to grow.
I think the high intensity classes seem to do pretty well.
The funny thing is, there are trends, and people do like to go try a new thing, but the classes that retain do not seem to me to be the ones that simply match the trends (which may come and go), but the ones that have the instructors that have the charisma, and teaching skills that just reach people and bring them back.