USD was and still use for how many centuries now and it's a currency of one of the powerhouse nation so I think it'll stay relevant even if Bitcoin is the future money. Local currencies is serving each country and Bitcoin's vision is not to eliminate it though. And above anything else every Bitcoin holder expect their investment to grow then withdraw it with USD 
Not every bitcoin holder withdraw their bitcoins with USD but they use their specific currency depending on their nation. I believe you that US dollars are been served since decades and if compared with bitcoins, it might gain more popularity and age but bitcoins are the currency of future. Peoples need to learn more about bitcoins as bitcoins might be the currency ruling the global markets in some coming months. US dollar are physical cash whereas bitcoins are not physical and it might be the only comparison between them atleast from my mindset but yet both are doing great job in their position. USD makes bitcoin trades easier and vice-versa Bitcoins affect the trades for USD.