anyone who has that much power will be able to reverse or create new coins out of thin air.
Any person with that much power will be able to retain the longest chain by himself. This means, that if he wants to reverse a block that was previously generated by the “honest” miners,
he can. Although, he can't create coins out of thin air, because of consensus.
You'll have to understand, though, that
most of the times, it's not worth to do that. You see, the PoW algorithm is written in a way to make it more profitable for someone to extend the chain, rather than to attack it. If the chain is attacked, then the coin's value will drop dramatically, since it's losing its point. Thus, he'll be left with some useless units instead of being able to generate
the majority of those coins.