But then some people can and do drop down out of A due to their ranking dropping below the cut line and not having qualified in A the year they moved up; one year and you can drop back unless you choose to play A anyway. The "A" restriction criteria may have more to do with the headcount in A than other things do.
All said, though, I can't see growing A much beyond where it is now. At Pinburgh, you play well, you get A. At PAPA, you choose ahead of time. If you're not in the top 200, I can see why people would rather play in B - - cheaper, better chance of winning something. Whether it's tickets OR Herb, I don't think many more people will try A than do now. Minor growth, yes, but nothing huge. Overall growth A+B+C+D, yes, but more in B, C and D.
What would grow A? Something where B players thought they had a better chance than now of qualifying and or winning. But that something would mean it's less likely that the cream rises to the top, which is what the current A players want. EPC's one-shot runs give B players a better chance, but I don't see that being popular here.
It would be interesting to see who played what if each division, including Classics, was 100% self-funded for prize money, i.e. all entry fees, of whatever different values, stayed within that division.