You write that the real value of biitcoin is the fact that it can be exchanged for something else,.. But according to me that's a circular argument:

You can exchange it for something else, therefore it has value. It has value, therefore you can exchange it for something else.

The first aqcuisition was a 10000bc pizza clearly the value of bitcoin was very different then.

I also have countless object around the house which I could exchange for something else. The only problem is no one else seems to want to exchange my dirty socks for their bicycle.

Somehow, people have come to accept cryptocurrency as a store of value, even though it is inherently worthless. (don't get me wrong, many stores of value like gold and diamonds have very little value if it weren't for a demand which isn't justified by it's economic uses)

That being said, once people started to accept bitcoin as a store of value, (and many consider it an investment) It became a bona fide store of value. Once acceptance grew, you also obtained genuine uses for it. Nowadays, You can buy a lot of stuff for bitcoin, and I can send money around the world with it far cheaper than by sending currency through a bank.

But it's value for sure is not that it can be exchanged.

I could "invent" a new altcoin similar to bitcoin right now, start mining it, and no-one would want to accept it if I didn't market it.

So my question remains: what creates the calue of bitcoin and other cryptocurrencies.