The value hasn't evaporated - if you bought a bitcoin for $1000 a few months ago and now you can sell it for $100, then value of ~$900 (assuming that 'real worth' of USD didn't change much) was transferrend from you to the guy who sold you that bitcoin back then.
You buy at $100 and sell at $1000. Where did the value come from that compelled the new guy to buy it from you at that inflated price? In my thinking, by your example, the new guy wouldn't pay an extra $900 because the value didn't already exist in the bitcoin you are selling.
No, initially you were in possession of the value, as you could have sold the bitcoin to somebody else for that $1000 dollars. The fall of value in a bitcoin did not somehow go back in time, find the person you bought it from, and transfer value to them.
Think of it this way, I buy a nice car for $100k. The person selling it gets $100k in cash, and I get $100k worth of car. A year later, I crash the car and it is now worth $1k in scrap metal. Where did my $99k worth of value in the car go? Did the dealer somehow acquire that value? No, I simply destroyed it. I destroyed the value of the car, it wasn't taken from me. The value of the cash I gave the dealer? That is more or less still the same, but that is his business, not mine.
Value is not zero sum. Creating and destroying value is trivial.
