I think TJ is onto it more than anything else... I started a new progressive not too long ago and filled it in one day (24 teams), and I've tried filling three themes since and one didn't ever fill, one I pared down to a 20 team league, and the other sat at 20 for almost two weeks before I got three signups today, so now I just need one more, but it's been about 6 weeks so far to get 23 owners in a league that is giving away $30-45 in prizes with no entry fees. In the past that would've filled in a couple of hours.
But the fact that I was able to fill a 24 team progressive in about 15 hours would suggest that most of the former theme league players have moved to progressives. I know I, like TJ, used to play 10-12 themes at a time, but now I only do 2-3, with 8-10 progressives and the occasional OL/CL.