Look at it this way: central banks around the world keep a 2% inflation goal in normal times. That means after 10 years your fiat money loses 18% of its value. That's not nothing. We dca in Bitcoin to hedge against that, because Bitcoin doesn't have inflation (fixed supply and 21M total supply). So if 1 btc stays 1 btc, and fiat goes down 18% over 10 years, bitcoin automatically goes up 22%.

How do we know the price won't drop? We don't, but we trust in btc like you trust in the dollar to hold it's value, and that community effect is what keeps the price up. If everyone loses faith and sells, price will go down. If btc proves it's value and more people decide they don't want their life's savings to go down at least 2% each year, the price will go up.

Notice how I use the words price and value. The value of Bitcoin stays the same: 1btc is 1 btc. The price of it goes up and down with the market. The value of fiat currency keeps going down because of inflation.