I too feel that the mixture of classic, post-classic and modern classes is spot on. At the moment the Manx GP is increasing it's visitor numbers and it's my belief that those visitors are staying for longer than the 2-3 days that most TT visitors stop for. The reason for this in my opinion is the variety. Every race day has a different character at the Manx whereas at the TT every race day is Groundhog Day.
Take away the variety and you risk taking away the incentive for people to stay longer. The longer people stay, the more they'll spend in the local economy and the more widely they'll spend it. That will in turn filter back into the Tynwald exchequer.
Going OT for a second, I'm also puzzled as to why the Formula Classic hasn't taken off. I'd have thought that there'd be tons of Tridents, Rocket-3s & Commandos lurking out there. Perhaps someone from the MMCC should liaise with the CRMC and BHR and see what will work - then stick to it!