A: Botnets can only easily gain control of computers run by those who are not technically savvy. Non-tech-savvy people very RARELY run powerful GFX cards. The Botnet owner would need thousands of computers in the Botnet to hope to achieve the power of a couple dozen powerful computers, or even a dozen duel-GPU systems.
That's a pretty sweeping assumption... Are you saying all gamers are tech savvy? I know plenty of die hard gamers that still think RAID-0 keeps their data safer because it's got RAID in the name.
B: Accessing the GFX card is one of the most difficult operations to do remotely, meaning this would only be achieved by a small minority of genius programmers capable of writing and constantly rewriting code to keep up with the nuances of security that gets in the way of accessing the GPU. In almost all cases, even these Botnet owners would be lucky to access even half of the GPUs in their Botnet.
bollocks... I can download, configure and start a GPU miner from the command line. From the command line I can check for signatures in the registry to detect the graphics card driver, I can download and install the appropriate OpenCL SDK. None of this is hard for someone capable of writing the code for a botnet.
C: Mining requires very high levels of GFX performance. The Botnet operator would need to either choose between having the GPUs in the Botnet run at full speed (causing both system slowness and crashes that would encourage the non-tech-savvy person to either buy a new computer or reformat the OS, likely increasing security as they do so, removing their computer from the Botnet. OR, the Botnet operator would have to run the GFX cards at a slow enough pace not to interfere with performance on these lower-end machines, which means conservatively 25% utilization at maximum to avoid the owners resetting their systems to factory spec and wiping away the Botnet software.
Detecting downtime for the machine is also not hard. Any botnet operator worthy of the name would understand the risk of running the slave pc flat out 24/7. Simply setting a low aggression with some kernels allows gaming without any noticeable impact yet when the PC is unused you get 95% of the hash rate you'd get if you had high aggression settings. Your talking 10000 but the biggest known botnet today is 4 million. Even it was only 25% utilization and they could get the GPUs working on only a fraction we're still talking massive processing power.
Between these points, it quickly becomes clear that a Botnet consisting of 10,000 computers will not be nearly as effective as originally considered. Point A takes the 10,000 and makes them as powerful as about 500 high-end dual-GPU systems, Point B drops this number to around 200, and point C drops the utilization to around 50. Assuming 500 Mhash/s per system, that is a grand total of 25Ghash/s for the ABSOLUTE BEST and well run Botnet. In all likelihood for the average Botnet owner (a few hundred systems) it simply wouldn't be worth the time or effort.
Even if 100 'Absolute Best' Botnets came into existence (or a few less with more computers under their control), a change of 2500Ghash is only 20% of the existing network today. Sure, these Botnet owners would make a profit, but if they all joined the network today (and none of them were previously in the network) we'd only see difficulty increase by 15-20%. And its fairly safe to say that there are substantially less than 100 Botnets capable of running 10,000 computers with the absolute best software.
This delusion, however pervasive, should be outright ignored. Mining will continue being hosted primarily by the network of GPU owners for the foreseeable future, so stop creating panic and paranoia.
You can call it a delusion but you've made some very assertive calculations based on completely false assumptions.