I am not completely sure that is the case. If you have two networks, one worth much less than the other, then the network worth more may have incentive to subsume the other network. I think this is what's happening now. And the same thing could happen to any network, even bitcoin, if a large enough actor wanted to subsume it.
The escape from that seems to be to fork and start fresh, on a new chain without that malicious actor. That may be feasible for very well-established networks, and not so feasible for not-so-established networks.
In short, this attack vector doesn't seem well enough explored. It was usually assumed that an entity buying into a network would have the network's best interests, but this may not be the case if the entity's larger vested interests are in another, more powerful network (like another blockchain, or a corporation, or a government).
If Justin bought 70M normal Steem (instead of that ninja-mine), there would not be any problem.
Have you looked at his previous acquisitions? Like: Former Employees Sue Justin Sun and TRON Foundation, Alleging Workplace Hostilities