In a way you could say that it is backed by Fiat Currencies...
Think about it you can buy and sell things with bitcoins and what do people do with those bitcoins when they have them? Buy things with them ? Maybe, but the majority of them will eventually exchange it for Fiat. Because lets face it in the physical world there's not alot of places where you can use Bitcoin.
Even though i think it's backed by Fiat I personally believe imho that this is still great. Because even though Bitcoin can not replace fiat it provides so many great features for E-Commerce. Bitcoin to me is Virtual Cash and I will be very happy to use in when it stabilizes in the end because of that.
All it takes is one or two supermarket accept bitcoin payment, nowadays large supermarket almost has everything people want in daily life. And the supermarket can pay their employee and sub-contractors with bitcoin
Fiat money has been existing for a long time, so most of people don't ask why it still has value after the gold that used to back them were removed 1971
If you are a merchant and everyone have both bitcoin and fiat, you will definitely welcome bitcoin over fiat, but people might want to spend inflative fiat and save deflative bitcoin, so anyway bitcoin will not be used at large scale, it's like some kind of high powered money which seldom used to do the payment, although it has the payment function
If bitcoin value always double each year, who will spend them? They will more likely to mortgage them and get a fiat loan to spend
In a perfect world Bitcoin would replace fiat. But I think it's gunna take more than 1 or 2 supermarkets for people to use bitcoins.