Proof of Stake requires that what you are staking has value. The future I'm looking forward to is POS taking over by leveraging the existing Proof of Work already invested in Bitcoin and Ethereum. So, Bitcoin will slowly go away by being burned to transmute it into the foundational value of new POS coins. During that process, the total supply of unburned coins will decrease. Eventually the market cap will decrease. Less POW mining will be necessary. And, eventually it will drop off to effectively zero.