How can you be sure that you won't wake up one day and your bitcoins will be worth nothing?
There is intrinsic value in the service (distributed consensus on a global ledger) that bitcoin provides. As such, the only way bitcoin could be worth
nothing is if some flaw were discovered that made it impossible for bitcoin to provide that service any longer.
Far more likely is that you could wake up one day and discover that your bitcoins are worth
very little (for example 10,000 BTC = $20). I personally believe that demand will continue to grow for what bitcoin provides. Since there is a limited supply of bitcoins, an increasing demand must lead to an increase in exchange rate. This is obvious from even the most basic understanding of supply and demand. I can't "be sure", but I am very confident.
Here's a question for you to consider...
How can you be sure that you won't wake up one day and your bitcoins will be worth $10,000,000?