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.
They could do it anonymously. The difference between steemit and reddit is that on steemit, a downvote from a high value account hits the recipient's bottom line. For example, if you're trying to launch a start-up music business on the blockchain, how long can you survive high-stake downvotes from the recording industry? They don't seem to mind the bad publicity that comes from takedown notices.
I hope you're right about the ability (and willingness) for another large stakeholder to nullify abusive downvotes.
We will have to see how it all plays out in the long run. :)