I know this is a joke, but what would prevent the thief from attaching high transaction costs to the withdrawal of ETH from the target wallets? Say they place 50% of the value into the transaction costs - miners would assume a healthy profit off of the theft and it wouldn't be beneficial for them to rollback the chain.
Takes longer for such a change to propagate than for the miner to just move his coins. Unless you implement it as some kind of history tracking for wallets, but then the hacker can just send a tiny transaction to literlaly every wallet around, and everone's paying the 50% cost.
A hypothetical hacker could robin hood easily then.. only steal coins from wallets with massive amounts, and "give them away" to miners via transaction costs
