Good example of simple != better statement. It will help you for one threat, but opens another hole. Actually, such fix is less secure than calculate block trust using an original algorithm. It can be forked without a significant part of stake or hashpower by running a parallel chain at lower PoS & PoW difficulties. Because it makes no difference between coindays consumed or hashpower wasted. One CPU is able to beat the entire network.
The same is true for Bitcoin. That's what the hardcoded checkpoints are for.
That's not true, it seems that you don't understand Bitcoin quite well.
1. Bitcoin is able to make a difference between diff1 or diff2 blocks, one diff2 block can't be beaten with one diff1 block.
2. Hardened checkpoints only purpose is to be a trigger for signature checking optimization and protection against compromised ISPs. And you can disable hardened checkpoints verification in bitcoin.
Fair enough. This will need to be changed, thanks.