Mining won't be controlled by botnets as GPUs will eventually be replaced by custom hardware (ASIC/FPGA). Difficulty will rise so high that having 20GH/s will be like having 20MH/s right now. Botnets will control a portion of mining for a short period of time, in relation to the BTC mining timeline. It's naive to think that GPUs will be the preferred method of mining 1-2 years from now.