Have you guys seen the participation numbers for the open-setup Proto/GT races this season? It's absolutely horrible, one of the worst in iRacing. Moving to a fixed only setup will improve the participation. I don't like the idea of 25 minute races, I would think 40 would be a little better, but it's proven time and time again that shorter races mean better participation.
As far as C-class and above drivers entering rookie races, I did it too, simply because it was convenient, not because I wanted to or because I enjoyed it. Having that option taken away will make me more carefully plan my entries into other series. This is a great move by iRacing to get people away from the most popular series and into series that are hurting for participants. I don't see why everyone is so upset, you can still race the MX-5, just do it in the GTC or iRNC. Or even better, move to a different series and enjoy different cars and tracks.