The fact that you think there are 20-50 botnets with over a million bots, shows that you don't know what the fuck you are talking about.
The incentives are just too high. 1,000,000 infected computers (not hard to achieve at all with a Botnet, in reality the more successful Botnets have 10 million or more) will drive the difficulty through the roof!!!
This cements you as an idiot. How many botnets do you think in the history of computing have had over a million bots?
http://en.wikipedia.org/wiki/Botnet#Historical_List_of_BotnetsI don't know anything about this but I can still tell this is nonsense. I don't know exactly what those existing botnets do but I am pretty sure they are about as intrusive as .bat loop with a ping command running in the background...
When you start mining on someone computer they will notice. It will be hot, noisy and slow. It is enough for most people to have there computer looked at. Or they will just buy a new computer, yes there are loads of people that just go and buy a new computer because the one they have is slowed down by malware...
Anyway those computers would be wiped or replaced much much faster then current botnets.