Interesting, wouldn't checkpoint servers and checkpointing every block combat this though?
yes, but it wouldn't be a decentralized currency! it would be a system controlled by a centralized checkpointer.
If you did decentralized checkpointing every block then the network would split and wouldn't work.
I don't think being fully decentralized truly matters, having a centralized checkpointer can often be better since there are many people that work in a decentralized fashion but could be trusted enough to manage the checkpointing. IMO a centralized checkpointer is better than a decentralized currency with an open attack vector.