G'day Jeroen,
I have to agree with some of the points you are making,when the drop back rule came in I thought it was a good thing, however, I reckon the class gap you are talking about is definitley causing some grief.
There have been many times lately where you see a horse with 9 or 10 life time wins, $45/55K in prizemoney to his name and he's in a C1 race at a country meeting where the opposition are typical country C1 horses, and guess what! he's showing $1.50 on the tote, stifling the betting and wins it with a leg in the air!!
I think some adjustment is required to the drop back rule, whether that is as you suggest restricting it to mares or whether it's adjusted to say C2 or C3, or the number of non winning races form part of the adjustment, I'm not sure, but I reckon you're on the money that its a bit counterproductive as it is.
Cheers,
Dan