Hi Julian Wilson, thanks for the feedback.
What I meant by “checkpoints is a centralized solution” is that they might be directly centralized, or implemented in a decentralized way, but will cause problems down the line that will still require centralized solutions. So in effect, checkpointing is centralizing.
There’s actually a difference between ‘old private keys attack’ & ‘current private keys attack’. You can check out my Part 2 which discusses this more in-depth.