That's not true in the least; in fact, nothing out of the ordinary would happen unless the attacker started abusing his power; he could just sit there producing blocks all by himself forever, taking 100% of transaction fees.
He can't just sit and produce blocks forever. In order to be able to produce blocks he must keep balances under his control. First he must exclude transactions emptying his cheaply acquired priv keys, then he will probably want to transfer balances from that keys to his own keys, because if he doesn't do that those gullible large stakeholders who sold him their empty keys will be able transfer funds from those keys again to their new addresses. In any case, attacker's fork will look completely different.