The money gets value because someone accepts it as a mean of exchange. Usually this happens within boundaries of an economic cluster. "Cluster" implies some degree of centralization, if extra measures used to counteract "buy keys" attack don't increase the level of centralization then the problem of centralization doesn't even arise.

Economic cluster works this way:
- Alice wants to buy something in Walmart
- There are thousands of different versions of the same blockchain
- She does the payment on the same version that Walmart sticks to
- She doesn't care about the other versions as long as she gets what she has paid for

So, as we see even "buy keys" attack can do nothing if economic majority keeps an eye on the blockchain and doesn't allow deep reorgs. This is what happens in Nxt with its 720-block rollback limit.

The OP should has included the above text to look non-biased. As a bonus extra analysis on possibility of an eclipse attack that could split the economic cluster and lead to chaos is welcome.