It's a valid concern. A couple things help me breathe easier about that, though.
One, STEEM is so rare. It's hard to buy a lot of it quick, and if you do it will get expensive fast. 98% of it is powered up, which leaves very little for the open market.
Two, If they do start buying it up like that, they are going to make us a lot of money ;)
I guess! He who holds STEEM has power!
I believe personally it's just a matter of time before we see incursions of these large media moguls into the STEEM ecosystem.
It's probably inevitable. They try to dominate every possible outlet. I hope we can resist it to some extent.