I agree that the point of a currency is a medium of exchange, and that's exactly why it isn't about investing. Investments are something you do specifically to earn money. Whatever I ever hold, I do it because I'll use it soon enough.
You might argue "but if nobody holds it, there won't be any value", which is true. There is no chance that'll ever happen, though, because maintaining coin value is in some people's interests, i.e. mining farms.
People don't usually buy US Dollars to uphold it's value, they just use it as a coin. But just like mining farms, there are those who work hard to sustain it.