So then we could call it the Amazon coin. Since they have datacenters filled with servers just sitting there waiting for there to be a need for them during periods of high use.
They could just kick them on to mine your PoS RAM coin.

There is no need to change the way things work now. You buy a miner and you mine. If you can't buy a miner you buy coins. If you change the algo from SHA256 to something else it's just a matter of time  until someone starts building purpose built miners and the people who can afford them get them and those that can't are back where they started. Possibly even worse off if they invested in something that is now obsolete.

-Dave