If you prevent botnets from mining (e.g. by requiring an unusually large amount of memory) you prevent ordinary users from mining and encourage centralized farms.
I said "significant fraction", not large majority. A memory requirement between 1/2 and 2GB will
make ordinary users with little memory suffer, and allow ordinary users with plenty memory to mine.
Both groups constitute a significant fraction.
Okay, but then botnets will just stick with machines having sufficient (free) RAM. They'll use their other (smaller RAM) bot machines for something else (possibly mining a different coins).
I'm aware that botted machine are disproportionately older and less powerful but not entirely. There really has to be a pretty large overlap between users and bots, otherwise bots will just disappear entirely as the older machines gradually and eventually go out of service.