With 1.9 million bots, I suspect that they can do better than hugely inefficient CPU bitcoin mining. And if that's not true, it will be eventually, as the cost (in CPU cycles) of mining gets higher.

Bitcoin mining has to be the most profitable (or at least close to it) use of a botnet for this to happen, and since the value of mining keeps decreasing this will by definition not be the case at some point (if it even is now).