> Isn't Bitcoin the only currency which is in fact backed by Nothing valuable
Depends on what you perceive as valuable.
Theoretically, it could be argued that BTCs value ought to be as valuable as the time, equipment and energy dispensed in mining it + a perceived premium or discount.
In that it is similar to most commodities and currencies.
Further, if someone(Alice) doesn't mine but rather buys bitcoins, then on an individualist scale the price paid is the notional value of the bitcoin's that they hold. Next, Alice just need one person willing to accept her perceived value in exchange of goods and services rendered.
> Paper money imo is backed by regional Economy
You're right. But the same may hold true for bitcoin in the future, if they can develop economies that accept it at all stages of production and final use. They only such economy that exists in part is for mining equipment. Unfortunately, that too is exposed to other(less volatile) currencies which render it inefficient.