Sure, all speculation, and no guarantee, but you buy insurance when PUTs are cheap... right?
The reason why people are sure is partly because of the price history of bitcoin. We see the cycles of adoption, and there is no reason to assume it wouldn't continue. Bitcoin is looked at like a network, so it benefits from network effects, just like the internet. The more people who use it, the more use it has, and exponentially so.
Bitcoin has many properties that people like, be it a store of value, or method of exchange, and simply your only way to save if you're not in the lucky 1B of people who have access to banking.
When you also look at the history of world reserve currencies, you see that USD is at the end of the cycle which generally happened every 100 years. The next currency will not be tied to a country. More and more people are learning that its pointless to do a fiat currency.
On top of this, the world is digital now, and exchanging value is at the heart of everything that is done in the world. Just like the internet is for the most part neutral, a way to exchange value all over the world also needs a neutral mechanism.
The list actually goes on and on, but these are just some highlights.