Proof-of-stake being complemented in Ethereum will be incredibly bullish for many reasons:
- There will be a moderate increase in demand for ETH since it will become an income generating asset.
- The ETH staked will be locked up and significantly decrease the circulating supply.
- Currently there is no supply cap on ETH and the rate of issuance of new ether via proof-of-work is extremely high. Once ETH transitions to proof-of-stake, the inflation rate will likely be less than 3%. It will even be possible to decrease the overall supply of ETH by burning transaction fees.
- Proof-of-stake is prerequisite for certain scaling improvements, such as sharding.
- The network will be much more secure since proof-of-stake is much more resistant to 51% attacks than proof-of-work.
In summary, proof-of-stake will increase demand, decrease circulating and max supply, enable critical scaling improvements, and increase the security of the network. Once it becomes clear that proof-of-stake will be successfully implemented in ETH, the price of ether will skyrocket.