[ye]

No, a botnet can't ever achieve 51%. One modern ASIC rig is equivalent to a few thousand average CPU+GPU computers that make up a botnet.

[jonknee]

What about a botnet of modern ASIC rigs?

[ye]

How would you get one?

ASIC owners are paranoid about their earnings. They would notice they are getting less than they usually do the next day after the infection.