USD price does fluctuate and inflate over time yes, but the difference is that this is essentially an invisible effect on day to day life for anyone that is not doing forex trading.

The reason the US dollar is "stable" is partially due to the fact that it is backed by the US government. It is a requirement that businesses operating in the US accept payment in US dollars. Until somewhere mandates that bitcoin _must_ be accepted, it will never truly be a currency, the same way that gold is not a currency, and the same way that wheat, or potash, or shares in GME is not a currency.

There's also a concept of "sticky" prices that most economists are quite familiar with - this assists in keeping the price of a coffee stable, whether or not technophiles accept it or not. Even if the price of the US dollar fell by 50% today, your coffee would still cost you around $2 or $3 tomorrow (assuming the coffee shop was even open).