On the other hand, if it works, then why does it have a retail price at all?
Most malware has a shelf life, depending on how fast the defences evolve to counter it, so any bit of code isn't a perpetual money machine, and crims need to get the harvest in while they can. Even if it still works, the advanced crims will be developing new versions, so selling a superseded version that the original "IP owner" is money for old rope from their point of view.
Also, ensuring there's a lot more low rent black hats and crims dilutes the ability of the plods to find the people at the top of the chain. Both because there's more crims, and because more of them will be unskilled dopes.