At least for Bitcoin, there's another defense: what the blockchain actually stores on each block is the root of a Merkle tree which has the transactions as leav

At least for Bitcoin, there's another defense: what the blockchain actually stores on each block is the root of a Merkle tree which has the transactions as leaves. It's possible to discard unwanted transactions, and that only loses the ability of verifying newer transactions which chain to these discarded transactions. (AFAIK, the original intention was that most nodes would discard old "spent" transactions, to reduce the size of their local database.)