So the 10 btc I bought a couple years ago for ~$2 are now "worth" $300 USD. Do I sell them now, or wait 20 years until they're worth more?
How do you know when to wait longer, and when to sell? Its not like a company where you can at least get some information on their past and future performance, whether they are growing or shrinking, etc.
The volatility of BTC is useful in such situations. If you sold now, the price may rise to $40, then crash to $15, and then rise again to new heights. So if you do sell (and are keen to stay in the long-game), you have to be confident that you will be able to buy in again at below that price (NB transaction costs). If you achieve that (which is done most effectively by guessing when the peak of the bubble occurs) then you wouldn't lose out it the long term. Then you have to weigh up the risk vs reward..
According to the efficient market hypothesis (which you may or may not agree with) the only guide you have as to how much something is worth in the future is the current price, as well as "future information" which is not currently available (if it was already available somewhere, it would already be priced in.)
In other words, no one can answer that question for you :)
How do you know when to wait longer, and when to sell? Its not like a company where you can at least get some information on their past and future performance, whether they are growing or shrinking, etc.