One way to compare Proof-of-Work to Proof-of-Stake is to look at the production of work through time that is required to earn future rewards.
In the case of Proof-of-Work, future rewards can only be earned by work provided in the future while for Proof-of-Stake, future rewards are earned through capital that has been earned and allocated at an anterior date to future earned rewards.
Proof-of-Stake attracts passive actors which can simply buy, stake and forget (especially in the case of delegated stake) while Proof-of-Work requires a continuous involvement of the miners to keep earning rewards.
The importance of not distributing rewards only based on past work is that doing so can give an enormous advantage to the “early adopters” (that may just have been at the right place at the right time) while discouraging future competition.
Of course, in Proof-of-Work capital expenditures are required and those are technically work that has been provided in the past but operational expenditures (which is work that has to be provided in the future) is still required. However, the more capital expenditure is required, the more a Proof-of-Work system is close in nature to a Proof-of-Stake system as the importance of past work is higher than for Proof-of-Work systems with low capital expenditure requirements.
Intuitively, we could say that #Ethereum (pure PoS) rewards past work more than #Bitcoin (high CapEx) which rewards past work more than #Monero (low CapEx). There are also other approaches that fits in between such as #Idena which relies on Proof-of-Human-Work (Lowest/No CapEx) coupled to sublinear staking (lowest staker earns the highest yield, highest staker earns the lowest yield).
Another advantage of distributing the rewards for future work is that it protects the network against seizures. Stake seizure by state actors are a threat to the future decentralization of a network while seizure of mining hardware isn’t in itself sufficient to pose a risk to future decentralization as some operational expenditures are still required.
Those thoughts on PoW vs PoS aren’t totally fleshed out and it would also be interesting to consider the impact of the ASICS performance race (constantly requiring new CapEx) as well as the choosen rewards issuance curve (early adopters receiving more rewards could produce a “right place at the right time” effect seen in PoS).
