This makes perfect sense this morning.

The granularity I was thinking about though, is "one thing" might be a specific change between two neurons which may relate to one memory for one perceiver, and if any one of those changes is reversible then technically many such changes would make even a whole memory reversible. Though it is punishingly expensive to do so...

Then comparing that to what may be required in the future to rewrite Bitcoin and are the two comparible in terms of time and energy requirements.

It wasn't considering "will someone be able to undo something very difficult" rather, would anything major ever be worth it (probably not), and is the genesis block on this ledger going to be more or less the most difficult thing to "undo" that exists?