How would they compete against all of the other large holders? Sure someone could buy a ton of steem power, but their downvote could easily be nullified by another large account. Were they to purchase and downvote content like that, it would look terribly for their business (or what ever organization they work for), and those they are promoting. I don't think that sort of behavior would end well for the multi billion dollar 'company/entity'. It's likely very complex, but I do not suspect it is steemit's downfall, any more than a multibillion dollar adversary buying a ton of reddit accounts and voting with them has been reddit's downfall.

I will add that this is an interesting thought experiment to consider.