> This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that p

> This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Not exactly, for every block you rewind there is extra work you have to do, to the point where it may take you close to 2 years to rewind 1 year's worth of blocks (because while you're doing that the rest of the network is still creating new blocks). It gets exponentially harder/more expensive the further back you want to go.