This is a great theory, but still doesn't explain why it haven't happened in reality. You can make up scenarios and theories all day of "what may happen", but when it doesn't actually happen in reality, you have to question your theory, not question reality.
Gavin already addressed your question about why it hasn't already happened in reality. The fact that an attack has yet to occur historically is no way to design a secure protocol either. The fact that you are even using this as an argument shows you aren't serious about security or PoS flaws.
Yes, in Network and Information security one must analyze and prepare for all hypothetical attack vectors. This is the reason most Bitcoin proponents who understand Information Security still suggest Bitcoin is "fragile", 7k average active nodes is too few, and how they would prefer other implementations like libbitcoin to exist alongside Bitcoincore that interact with the blockchain.
There's no checkpoints in Bitshares, users will be able to opt out checkpoints in Peercoin 0.5, and I don't know enough about NxT checkpointing to comment.
Bitshares PTS is DPoS thus is centralized amongst delegates by design. The criticism of checkpoints lies within the fact that they depend upon centralization which introduces an added security flaw above PoW.
You are speculating about future changes of Peercoin and Peercoin is a PoS / PoW hybrid anyways so can always fall back on PoW for added security without checkpoints.