but nothing has been done about either adjusting to sharp changes in hashpower, or preventing rewriting the chain with big hash power in private then dropping it on chain and reorging to a censored ledger. bitcoin is one of the only PoW coins with enough resident sha256 miners to not be vulnerable to substantial change, proportionally the cost of more or the cost of not running it are both small. but as i point out, for any given alternative proof of work hash function, the amount of potential hashpower on the market is now insane. GPUs in massive clusters can easily be rented. spiking the hash power or using it to change the recent blocks and remove transactions quite a long way back are both feasible now with any hash function. sha256 isn't the only function with massive potential attack capability.