Maybe a decline in account farmers would also mean a decreased payout for signature campaigns, or even fewer campaigns in general - which would probably be good.
I doubt if that would be the case. If there is a decline in account farmers, my guess is that you would have signature campaigns paying higher for the remaining participants. Sites would anyway discount alt traffic when they take into account whether they want to advertise here or not.
I agree with botany, campaign owners will be willing to increase pay to make active campaigners to switch to another campaign because of the higher pay and with higher pay, this would also force the campaigners to make sure their posts meets the quality standards to avoid being kicked out of the campaign or posts not being counted.