they cannot without expending large amount of energy and even then it will look extremely shady to both old and new nodes. in pos you can generate an entire new

they cannot without expending large amount of energy and even then it will look extremely shady to both old and new nodes. in pos you can generate an entire new blockchain from genesis and new nodes won't have a clue which chain is "real" while old nodes will be very much confused having to make a decision on the split and reorg. point is - pos is based on trust, pow is based on laws of physics.