Although there’s no way to know exactly if and when an alternative to proof of work will become necessary due to a lack of mining incentives, a proof of stake scheme could also be a desirable solution for environmental and efficiency reasons. Since the proof of work process does not actually solve real-world problems, the energy is essentially burned without a real return, which is suboptimal. Implementing PoS, either in the form of a fork from the main proof of work blockchain or via the use of an altcoin that uses it (ie Peercoin, or something similar) could be significantly less costly than bitcoin mining as it currently stands, because the current system gobbles up a huge amount of electricity. Because PoS uses far less energy, as almost none is expended in the mining process, it would be substantially cheaper to make a profit mining than in a PoW scenario. It would also meaningfully reduce transaction fees in the long run, as miners wouldn’t have to charge high fees in order to cover their power and hardware costs.
We still lack a perfect solution to all these issues, and PoS is not a panacea either. One problem I see with implementing a PoS mechanism is that it could cause illiquidity in the market and lead to great concentrations of wealth. Miners would be incentivized to hold their bitcoin in order to be allowed to mine more, and therefore large concentrations pools of currency would accumulate. Currently, miners have an incentive to convert some of their mined bitcoins into dollars by selling them, but this is largely true because of a) price volatility – it is still risky to hold everything in bitcoin and b) there are still many assets that cannot be purchased using bitcoin. If PoS were implemented, and as both a) and b) become less relevant as the network expands, this could lead to a vast majority of coins being held by very few.