RAM is actually incredibly cheap. 2GB costs ~$10 and that price will be half that in 12 months. While server ram is more expensive it is much cheaper than multiple complete computer systems. For example 1TB of FB-RDIMM runs ~$30K. While that is some serious coin it enough for 500 instances. $28K is far cheaper than 500 computers. The per instance cost would only be $60. As a % of overall computer cost RAM has been falling over the last two decades.
As a commercial miner I would love you "solution" I could replace my entire jury rigged GPU farm with one rack of high density servers and put them in a co-location cage. The largest risk for me wouldn't be legitimate nodes it would be botnets. It is hard to compete with $0 cost.
Still I don't see what "problem" having a cpu-only work unit solves. If anything it makes the network MORE vulnerable to botnets. Most users will simply not hash if the reward is ~$2 per year. So you put a limit on how decentralized the network will become. Those users will likely joins pools so pools which already the largest threat to decentralization still remain an issue. The network will never be decentralized enough to be immune to botnets. My prediction is within a year the current CPU-only alt crypto currency (can't remember the name) will be dead or forked due to the ease of taking over the network.
So looking at cpu only vs open network (CPU, GPU, FPGA, ASIC, etc)
1) it is more decentralized - dubious value see next points
2) highly vulnerable to botnets. Even 100K "valid nodes" would easily be crushed by Storm Trojan (230K average controlled nodes). A smaller network could be crushed within days. Even if there wasn't a financial incentive someone could hash the difficulty up to 1000x current and then leave letting the network fail.
3) unlikely to become "super decentralized" due to lack of financial incentive (most users won't hash 24/7 to earn $2 per year). There are many more potential users of bitcoin than potential hashers. Most people just want something with low fees they can use to safely buy and sell stuff. They have no interest in becoming a payment processing node.
4) Still possible for commercial miners to game the system by exploiting whatever combination of CPU/memory gives them the highest return
5) Pools still remain the largest vulnerability.
So what exactly does "CPU only" hashing achieve other than decentralized network for the sake of decentralizing? Sure I grant you a cpu-only network would be more decentralized than an open network however I argue that the amount of decentralization you would gain solves nothing and makes the entire network more vulnerable.