so when you unsub it just stops. There's no exit survey as far as I know. Numbers of people unsubbing matter but providing further information harms NOTHING and provides further feedback that may or may not be read.
I remember when I briefly unsubbed, there was a "pick from among these" list of options to say why you were doing so. I don't think there was any "other" field to give your own response, however. Not sure if it's still like that.

The forums are not just for the other players; they're for the devs to get feedback as well. At least, they're supposed to be.