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.