I believe I covered the re-org depth mitigation in the OP? It doesn't help for two reasons:
1. The re-org from this attack could easily be less than the maximum depth
2. Re-orgs greater than this depth will still be accepted by all syncing nodes, and the the attacker can impersonate a majority of nodes for ~0 cost, leading to all syncing nodes accepting his version of history, which eventually leads to it becoming the canonical chain.
Reorg depth limit is one of many ways to do the job. Economic cluster participants could use something else.
If you can describe one which doesn't involve something related to a checkpoint, or human intervention, I'd be happy to add that to the OP.