Ethereum lowered the issuance rate dramatically when it changed to PoS. The reason it was able to do this without lowering the cost of a 51% attack is that it

Ethereum lowered the issuance rate dramatically when it changed to PoS. The reason it was able to do this without lowering the cost of a 51% attack is that it no longer has to rely on rewards alone. It can also apply penalties. Certain provable attacks result in automatic destruction of stake. In PoW terms, it's as if a 51% attack caused the attacker's mining rig to burn down.