People may be able to trust that fiat is entrenched and ubiquitous, but they can't necessarily trust their government or their central bank not to devalue their fiat currency over time. They can't trust that billions of additional new units of new currency won't be magicked into existence from nothing. They can't trust that their nation won't be subject to embargoes or sanctions from other nations, putting restrictions on where they can or can't do business. Bitcoin has answers for all of these things and more.
But then what about the fact that if the demand for bitcoin one day drastically decreases, the value would plummet too?
I can't presently envision a world without the problems I've described. In fact, I suspect it's going to get a lot worse. If that's the case, it's reasonable to assume demand for Bitcoin will never disappear. Basing economies on perpetual growth and 'feeding the debt monster' is unsustainable. It invariably leads to eventual collapse. Bitcoin isn't debt-based and has a predictable supply.
Maybe if you think world governments are ever going to get their shit together and actually build an economy that works for everyone, I can see why you might be concerned. But it sounds like a fantasy to me. They just repeat the same failed concepts over and over. At least we're trying something different.