The valuations aren’t based on the notion that BTC will replace the dollar. Rather, they are based on the notion that BTC is superior to the dollar (and gold) and has value. That value is based on a number of things Including its scarcity (value is always derived by what people are willing to pay for it).
I’m not very good a math but...
7,500,000,000 people in the world.
Lets say just 1 percent adopt bitcoin as a store of value for a variety of reasons, including the fact that it stores value better than their native currency or the global reserve currency because it cannot be depreciated on a whim (to find a war, bailout a bank, absolve student loan debt, etc.), its censorship resistant, etc. That’s 75,000,000 people on the demand side.
Right now, there are only 18,000,000 BTC available for purchase. Of course, you can buy extremely fractionalized shares of BTC (aka a “Satoshi”)
Now imagine that 25% of that supply has been hoarded by a relative small group of people, leaving about 13,500,000 left for the demand side - the mere 1% of people (75,000,000) in the world who think its valuable.
Now imagine the price goes up when demand exceeds supply.
I’m not an economist but...