The thing is, I think most of the scammers are still here. They are just willing to sacrifice certain accounts out of their collection. One of the easiest ways to identify a bought account is by looking at the post-to-merit ratio. If its really high, this means they probably run a bunch of alt accounts simultaneously, so they never write anything worth meriting.
If someone has multiple accounts and all are in campaigns, they have to complete their daily post quota. Which could be the reason they don't have much time to spend on reading before they write. So, you have a good point and I think you are right with the assumption. But this point also proves that the point I mentioned is valid as well. If they do not earn merits, then they are unlikely to get a spot in signature campaign. Since the disappearance of the mixers, the competition becomes more tough for them. So, it is like selling the accounts for a few bucks!