So then, is the implication that any medium of exchange or any store of value is valuable simply because everyone has agreed this is how it will be?
For bitcoin there is a base intrinsic value because it is used as a digital currency. For example if I pay for some widget online with bitcoin. I had to go and find a bitcoin and pay for it. I then assign it to the merchant. The merchant may sell the bitcoin immediately and convert it to cash. Because bitcoin can be used in this way it creates a base level of demand for the crypto-currency. Since the hash values that represent the bitcoin are finite and becoming harder to find. The hash-keys that are in use may be fractionally spent, allowing a single bitcoin to have multiple owners that own a share of bit coin. This also drives the price up because as each person's risk is over a smaller portion of a bitcoin the less likely they are going to rush to sell their stake.
Because there is a market established where anybody can buy and sell the product demand increases as people speculate, just like what happens in the oil market. I'm not saying that Bitcoin is a safe investment. But if you had $7k of risk capital on hand I wouldn't fault you for buying a Bitcoin.
Many people are fearful of bitcoin because you need an electronic wallet on your computer. But you can buy a specialized thumb drive (hardware wallet) that will store your bitcoin offline until you are ready to use them. This means that as long as they are not plugged into your computer they are safe from electronic attacks.