How do you know which checkpoints to trust, unless they are signed? And if you have trusted checkpoints I fail to see the point of proof-of-stake (or proof-of-

How do you know which checkpoints to trust, unless they are signed? And if you have trusted checkpoints I fail to see the point of proof-of-stake (or proof-of-anything really). Just call every block a checkpoint and you're done. Any remaining use cases probably centers around availability, but we know how to build highly available systems and can build them to any degree. Trustless and decentralized systems are much harder.