It's very incidental, though, and most of it could be purged with mild effort.

The whole point of a blockchain is that it's append-only. Each block has a hash of the previous block. If you wanted to modify a Bitcoin block from a year ago, you'd have to do the same computation as a year worth of all miners.

The blocks are hash trees, and you don't need to store the whole tree. Purging depends on that, not altering the block history.