Spawn Camping Attack
"In a proof of work system, if your chain gets 51% attacked, what do you even do? So far, the only response in practice has been "wait it out until the attacker gets bored". But this misses the possibility of a much more dangerous kind of attack called a spawn camping attack, where the attacker attacks the chain over and over again with the explicit goal of rendering it useless.
In a GPU-based system, there is no defense, and a persistent attacker may quite easily render a chain permanently useless (or more realistically, switches to proof of stake or proof of authority). In fact, after the first few days, the attacker's costs may become very low, as honest miners will drop out since they have no way to get rewards while the attack is going on.
In an ASIC-based system, the community can respond to the first attack, but continuing the attack from there once again becomes trivial. The community would meet the first attack by hard-forking to change the PoW algorithm, thereby "bricking" all ASICs (the attacker's and honest miners'!). But if the attacker is willing to suffer that initial expense, after that point the situation reverts to the GPU case (as there is not enough time to build and distribute ASICs for the new algorithm), and so from there the attacker can cheaply continue the spawn camp inevitably.
In the PoS case, however, things are much brighter. For certain kinds of 51% attacks (particularly, reverting finalized blocks), there is a built-in "slashing" mechanism in the proof of stake consensus by which a large portion of the attacker's stake (and no one else's stake) can get automatically destroyed. For other, harder-to-detect attacks (notably, a 51% coalition censoring everyone else), the community can coordinate on a minority user-activated soft fork (UASF) in which the attacker's funds are once again largely destroyed (in Ethereum, this is done via the "inactivity leak mechanism"). No explicit "hard fork to delete coins" is required; with the exception of the requirement to coordinate on the UASF to select a minority block, everything else is automated and simply following the execution of the protocol rules.
Hence, attacking the chain the first time will cost the attacker many millions of dollars, and the community will be back on their feet within days. Attacking the chain the second time will still cost the attacker many millions of dollars, as they would need to buy new coins to replace their old coins that were burned. And the third time will... cost even more millions of dollars. The game is very asymmetric, and not in the attacker's favor."
