I've seen loads of topics of people trying to sell things and saying I want x amount of dollars worth of BTC. Surely if someone is so keen on the idea of Bitcoin fiat shouldn't even be mentioned.
Because BTC is a volatile currency, but US dollars aren't.
LOL. Right. How much value has the US dollar lost in the last 100 years? The USD is only seemingly stable because of the establishments in place to keep order and keep stacking the house of cards. BTC has been volatile in the past, but for the most part we enjoy great stability now. Depending when you bought in, you're still several hundred percent up.
Going back to OP though, the price isn't that important. The thing is that just like gold, providing a specific amount of weight doesn't tell you much. But comparing it to a currency gives a user a more "local" impression of the value.
In the future that could change, but we will see.
Most people do not keep cash fiat in this fashion for 100 years. They have no reason to.
Someone that has put their fiat USD into an investment that is "designed" to be kept there for a long time (eg stocks, long term bonds) then would have done very well over this 100 years, even after accounting for inflation.