I'm mostly puzzled by the fact that someone would let $1 billion sit in a wallet without earning interest...
[0] I know I'm oversimplifying here, there have been ups and downs in price, etc
I speculate that in the future, Bitcoin might become valued so highly, that people don't even bother using it in favor of other coins, and as people don't use it, no one then accepts it, then it loses value, and by then people are accustomed to using other coins, and Bitcoin dies. Who knows, I probably won't live long enough to see it.
This is similar to Gold with the difference that it still holds value, it is so expensive that usual shops don't accept it though but still has high value, am I missing something?