Because only stupid people buy low-end computers. And only stupid people are part of botnet.
you cited the wrong reasons, and the wrong reasoning, but yes, pretty much with a few exceptions you can expect botnet machines to be below average.
I don't see necessity for bulletproof server. For Pool? For C&C? Both can run on either VPS or .onion.
yes. you dont even need centralized c&c these days. it depends on the specific botnet. all i said is that bulletproof costs more than $300 if that's the route you will go.
I tell about 30 to 40% might notice slowdown and do something about it. Remaining 60% are so dumb that nothing will make them think about something being wrong. Including constantly opening/closing DVD tray. Still the possibility to lose 30% of bots is idea many bot herders don't welcome.
you can expect botherders to be VERY fucking pissed if you make them lose bots
Mining will not kill. 51% will kill.
yes, you expend a sizeable chunk of your botnet (which would otherwise be making A LOT of money) to 51% the network with unsure results, at the cost that you may lose a good portion of those bots. good job, you just put your botnet to the worst possible use you could ever put it to. i'm not saying a few people in eastern europe & other locations couldnt do it, but rather that atleast tentatively they have no specific reason to do it, or even to cooperate in letting/making it happen.
again: the botnet threat is grossly over exaggerated.