The same botnet can back an infinite number of these ransom notices, because when you get one of these ransom notices you have no way of knowing if the ransom notice you got today is the same botnet you paid off last week but with a different name.
Add in the fact that there are botnets available to rent, and you create a near-infinite supply.
Sure thing. But that is allways the thing with ransom. You don't know if paying once is enough. But also the criminals know, there is only a certain amount of money to be extracted from their victims.
IRL, if another gang threatens me, I can go to the first I paid protection for, and usually they try to convince the other gang that, no, they should not do that.
Ransomware hackers have call centers to teach people how to buy bitcoin. I'd suspect groups involved in this kind of scam would have something similar, isn't it?
Add in the fact that there are botnets available to rent, and you create a near-infinite supply.