Well in theory that could happen if people only used bitcoin or crypto for transactions, but that's likely to never happen, just as the entire world doesn't just use one currency. Also you have to remember people are likely always going to trade it for something else whether it is fiat or another crypto and even fiat currencies are traded against each other and lose or gain value over time due to numerous factors so they will likely always co-exist together.
I am not sure if this exist now but do you think that a time will come when bitcoin will be used as payment for service? Like what is happening now but instead of paying in fiat money people will be paid in bitcoin instead?
Um. Seriously? This already happens. Some people who work for crypto businesses get paid directly in bitcoin. Just check out the Services sub board here. It's an entire board dedicated to people offering their skills or work in exchange for bitcoin. Even you could do it. Many people even get paid by signature campaigns here and those are people who are getting paid to offer their advertising/promotional services. Not sure how effective they are to the people paying them but they certainly seem to be popular here.