PoW helps alot in protecting the past / giving us credible guarantees about immutability. Are there some scenarios or trajectories you see which might endanger the immutability guarantee? One thought I've had over the years is this: what if in the future the hashrate is that much greater that it would be trivial to reconstruct a different past because the hashrates of the past were puny. A few seconds could create the same amount of PoW that took decades to generate in the past. Then again to construct an alternative past you've got to remake all the more recent blocks as well which would be more difficult so it's probably not feasible but it's been a thought that has occurred a few times over the years.