That is only a problem while "PoC hashrate" is low; if it was carried out in industrial fashion like mining, legit actors would operate such high capacities that botnets would not have a major influence.
Would need to be extremely large datacenters to avoid a botnet attack as everyone has a hard drive in their computer. At least with memory hard GPU mining a botnet would only mostly exploit CPUs instead of high end GPUs.
Doesn't industrial fashion like mining undermine the whole reason of using PoC in the first place? What specific problem is being solved here?
Here are some basics for people to investigate before suggesting other consensus mechanisms -
https://blog.sia.tech/fundamentals-of-proof-of-work-beaa68093d2bhttp://www.truthcoin.info/blog/pow-cheapest/http://www.truthcoin.info/blog/pos-still-pointless/