Serious question…why ever sell Bitcoin for cash?

I’m a new investor for Bitcoin; however, I am starting to not understand the purpose of cashing out Bitcoin early.

Doesn’t it seem likely that Bitcoin is slowly taking over cash anyway? If so, then why not continually to invest and hold forever?

Whereas, if the currency switches to Bitcoin then people will have to buy more Bitcoin again at a much higher price. I can see the benefit for those that have millions they can convert to cash but isn’t that sketchy to do anyway?