At some point we will no longer recognize Bitcoin for what it used to be, and the idea of keeping any promise would vanish into oblivion. Would you trust the money system that is constantly changing not keeping the original promise? Sounds a lot like today's politics to me.
You have to take into account simple economics and numbers. They don't lie.
Bitcoin and other PoW coins are not efficient to mine - if the numbers don't work out and they are not profitable, eventually there will be no more value in these coins.
Simple as that, numbers and facts don't lie.
Well, the system was designed to be self-balancing, so no worries with the numbers here.
If it's not so profitable to mine, then miners will switch their rigs off, which in turn would lead to decrease in difficulty until it becomes again profitable to mine, and so on.
I think the question of PoW vs PoS is more fundamental. Would you like work to create money or would you like money to create money? I don't have a very good answer to that, but the latter sounds a lot like what we have today with fiat and the former sounds like what we should have in a resource-balanced economy.
Also, check out this thread about PoW, it might be somewhat relevant:
https://bitcointalksearch.org/topic/an-answer-to-perceived-uselessness-of-pow-hashing-neutrality-855520
