Something I've always wondered

I've heard alot of speculation from the crypto community that at some point in the future crypto will overtake or rival fiat currency as the predominant store of value and medium of exchange. But one thing I've never really understood is - if the value of a cryptocurrency coin is determined by how much fiat you can convert it into, how will it ever exceed fiat in transaction volume or "market cap" in a global economy? Ultimately people will always need fiat to pay their taxes so it's never going away right. And as long as the quantification of value of a crypto coin derives solely on a numerical fiat amount, how will that one way dependence ever allow crypto to overtake or exceed fiat? Of course that doesn't take away from the huge value and likely rise of De-Fi and NFTs etc but does anyone else wonder this or am I thinking about it wrong?