People look at things the wrong way. Let's use a real-world example to show it:

Gold was a form of money. Silver was its lesser counterpart.
Fiat was created, based on gold. Now fiat is the main currency, while gold/silver are investment vehicles.

All of these have value. They are all convertible between one another. Just because fiat took over doesn't mean the others were destroyed/worthless.

I think BTC is the same way. I have very little doubts that it will be taken over by other currencies. But I think it will remain a main investment vehicle and therefore still have its use/value. It just won't be what we use for day-to-day transacting.