I'm sure that many people will choose BTC on top of the USD because of the many benefits it provides. Despite lacking mainstream acceptance from businesses and merchants worldwide, Bitcoin has been able to remain resilient and censorship-resistant after all these years. It shows us that it'll be here to stay for a long time. Even if it doesn't manage to scale properly in order to be used as a currency for daily transactions, it'll serve well its purpose as a store of value. The biggest benefit of Bitcoin is that it's largely divisible, making it useful for sending minuscule amounts of money. That's not widely possible with the USD, since it's only divisible to two units (making it terrible for sending payments that are less than a penny).

Among several other benefits Bitcoin provides over the USD, we have the ability to transport our money anywhere in a safe fashion. You could easily store your Bitcoin in a very small piece of paper or even a microSD card, with no clutter at all. With physical cash (which is the USD in this case), it's not possible to store thousands of dollars within a small space.

Still though, Bitcoin is missing the most important thing in order to be able to compete against the USD. Without being stable in price, it cannot compete against the USD directly, as it's terrible as digital cash. For that reason alone, many merchants have been hesitant in accepting Bitcoin for daily payments. Other than that, if Bitcoin would've been stable and highly scalable, I'm sure that the USD's existence would've been threatened by now.

Nonetheless, the choice of USD or BTC depends on the individual himself. For some occasions, the USD is necessary in order to transact with most merchants worldwide. But for others, Bitcoin is essential in order to maintain complete control of your money without fear of someone else taking custody of it. Which is why, both currencies might co-exist alongside each other for a very long time. Just my thoughts