For your second point, we need to keep in mind the free space might be empty but it is not free as in beer. Using the disk will cost extra electricity and CPU cycles. Plus we shouldn't forget that disks are still the most common piece of hardware to fail inside a computer. Increasing usage of that disk increases the risk of failure.
The pricing is left fluid precisely to avoid this. The price can change over time. 1 Filecoin / N time today. 0.1 / N tomorrow.
> Using the disk will cost extra electricity and CPU cycles.
Similar to how mining works today, but more useful. Filecoin today still wastes some CPU. But, we've got more surprises coming soon ;)
That makes sense from an economic perspective but how does that work from a customer service perspective? Is the price pegged to a different currency? How would a user be able to predict the cost of the service if the price is not pegged to anything?
Although in fact the decline of the value of the dollar is relatively predictable because of the Federal Reserve's policy goal of keeping inflation relatively low and the law of large numbers as applied to the generation of the real rate of inflation.
At my local deli a dollar used to buy a bagel with the works; and now buys less than half of a plain bagel. That's over the course of 20 years. Any unit of exchange is valued anew in each transaction; the persistence and trends in value exist mostly due to social pressure and the cost of bargaining. I don't make offers and counter offers on each bagel I buy; I just look at the sign and accept the price or leave without a bagel.
That's a crucial point. If the Fed didn't pledge to keep inflation under control, the dollar wouldn't be such a great store of value or medium of exchange. It's the stability promised by central banks that allow you to "predict the value of a dollar" in the immediate future.
That certainly hasn't dampened the investment possibilities in actual, government-issued currencies.
You can't peg a decentralized currency. You need to be able to control supply in order for the peg to work (ie. to sell more into the market to lower the price, if needed), and then it wouldn't be decentralized.