I'd love to see a proof of that. Not meant as a challenge and I don't necessarily disagree at this point. It just seems hard to say that because we don't know of a way there can't be a way, and such a proof would be interesting.
This is no proof, but you can say for certain that the cost of executing a double spend in any POS system is a simple constant proportional to the amount of stake you control. In POW, the cost is super linear in the number of blocks, which is far better security.