The answer is that there are other factors outside of the scope of your defined economy at play.
Just like in Bitcoin, my free CPU cycles are better kept free because if I try to mine Bitcoin I would have to leave my computer on 24/7 and it would cost me more than the BTC rewards I'd very rarely receive?
We've literally just swapped free CPU cycles for free disk space, and we both know PoW is centralized and it's not worth individuals with a laptop to mine, but somehow the free disk space won't suffer the same fate?
edit: Also even if FileCoin doesn't throw off a ton of ETH, presumably there will eventually be a wide array of services like the FileCoin network that you can transparently trade your liquid crypto assets into. For example, consider a person who happens to have 50TB of disk they don't need for the next month but would like to perform some low latency rendering jobs on the RenderCoin network during that time instead. They could easily just trade one for the other, increasing market efficiency for those resources. This example would be a small transaction in terms of ETH, but multiply that across the entire set of all computing resources, across all services, and it is a potentially tectonic shift in effective allocation of resources.
I haven't made the analysis but maybe Airbnb works because it is actually more profitable for an entity to host 100 airbnbs then to have a regulated hotel with 100 units, in which case the analogy doesn't work because that's the crux of my argument.
In the case of decentralized storage, this is not the case, as we've seemed to have agreed. Which means this:
1- The 4 data centers will host 99% of the available storage because it's so profitable for them
2- Because the supply of storage on the network is so high, the reward given to the hosters goes down (this is how FileCoin works I'm guessing, the reward needs to come down to close to the overall cost of the suppliers, like difficulty adjustments in Bitcoin)
3- Because the costs of the data centers which now have millions of disks all under centralized electricity/infrastructure is so low, the reward becomes lower than the cost of being an individual storage supplier.
4- At this point, the individuals with "latent resources" have no incentive to put these on the network, and actually are incentivized to sell the disks instead, because at least that way they'll make money.
5- Who is incentivized to BUY these disks? See #1
That's how economies of scale work and why they will make FileCoin useless.