What has supply growth anything to do with how the network is secured (PoW specifically)? There is no correlation nor relation whatsoever between the two.
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.
Security is strongly correlated with issuance i.e. supply growth. If miners must spend X amount of value to compute hashes and secure the network, and the issuance or protocol reward is not sufficient to justify this spending, the security of the network is at risk.
