Turns out the attack isn't possible. The assumption that miners can always mine empty blocks that the main chain has to accept is mistaken. Developers would release a full node update in response that rejects empty blocks based on some criteria of and there would be a network partition if this was ever attempted.
The good news is that it means no one can perform this attack, so we won't be at risk by some competitor or mining pool creating a bitcoin 2.0 blockchain that destroys us. Unfortunately this also means that bitcoin will probably never be able to upgrade its consensus algorithm away from POW, and will have to become irrelevant eventually, which I think is a shame.