Your reply and Jerrrrrrry's closed this understanding for me.
The attack itself can't be mitigated because there's the incentive to try to force the blockchain with your own theft block because your fee is much higher for what appears to be the same transaction. But this attack, like you said, is only feasible for this niche domain of low entropy private keys.
The bitcoin protocol isn't fundamentally flawed, but it is fundamentally outdated. If it wasn't for public bitcoin/crypto FOMO, bitcoin would have been deprecated years ago.