...Yes, it would take some time if the Ethereum stakeholders choose to set up the farms themselves, but it can be be done, even if it will take several months....
YEARS not months. That much excess mining gear does not exist, so you would have to have been stockpiling for at least a year if not 2 or 3 to get that amount of gear. Or you could just take over the used markets and buy it all but then you are suffering from having an mixed batch of used equipment you have to deal with.
Fair enough, let's agree to assume that it could take years (i.e. given that the existing Bitcoin miners are somehow very loyal and cannot be bought by the competitors/attackers).