One issue that proof of stake proponents often ignore is that proof of stake has its own costs that are roughly equivalent to the dollar cost of PoW. The issue is that you need to lock up money for PoS in order to mine it. That has a cost as you are forgoing interest on lending that money to an actually productive enterprise that is creating value. The net result is that funding and investment becomes more scarce because private investment is "crowded out" by people simply staking their Ethereum. This is the same harm caused by too much government debt "crowding out" private investment.
Also, if the PoS staking rewards are too low, you are vulnerable to the exact same 51% attacks as PoW. So it's very unsure how this would significantly reduce fees unless you are also significantly increasing your vulnerability to attack. (Now, there are caveats about the fungibility and the ease of renting PoW miners vs renting PoS tokens, but many of the same principles apply).