Scarcity alone isn't what'll maintain value though, I could create a coin that only I could mine, yet I am sure it's value would be stuck at exactly nil.
Some guy infamously referred to mining (proof of work) as proof of pollution and proof of stake as proof of plutocracy. Cryptocoins were created as the antithesis to plutocracy.
ETH being minable is what gives it value. Also, some other coin would jump into the gap created by ETH, so in that sense ETH is replaceable, but mining itself isn't.

SelayaScarcity alone isn't what'll maintain value though, I could create a coin that only I could mine, yet I am sure it's value would be stuck at exactly nil.
Some guy infamously referred to mining (proof of work) as proof of pollution and proof of stake as proof of plutocracy. Cryptocoins were created as the antithesis to plutocracy.
ETH being minable is what gives it value. Also, some other coin would jump into the gap created by ETH, so in that sense ETH is replaceable, but mining itself isn't.

Absolutely. The value of "normal" money lies in the fact that many people use it all around the world, and so products and services have an established value (even though that fluctuates with inflation, but that's a different story). If only a handful of people use a (crypto)currency, then it has no established value outside its community, so its usefulness is questionable at best. You need a certain number of people to use your currency for it to gain any value. Then, as more people mine with more advanced hardware, mining will eventually return lesser profits per node due to mining difficulty increasing with the amount of total processing power available (somebody correct me if I'm wrong). Then after a point, the hardware requirements of mining will be so great that even the largest mining farms will enjoy diminishing returns - hence proof of stake, which makes sure that the rich continue getting richer through being able to stake more cash. In this sense, mining is replaceable by PoS after a certain amount of capital has been accumulated, but the system as a whole will never be any fairer, just maybe a bit greener.