in any active network you are going to see lost coins; from people sending to incorrect addresses, to people losing their .dat files, etc, so if you have 0 rewards then it essentially means that the currency would be deflationary, and people have no incentive to stake.  So all you have to do is have a staking reward that is just high enough to make up for lost coins, the overall supply won't increase but people will still have incentive to support the network.  Monero's (PoW) planned tail-emission I believe is designed for this exact situation, I'm sure you could do something similar with PoS.