What you say has a point but it is not 100pct true.
Fiat even though is not fully backed by gold but it is at least determined by the strength of the economy, military to certain extent, availability of natural resources etc...of the issuing country based on iou trust

Bitcoin although is not backed by anything but it gets its value based on acceptance. .. the acceptance by the community who believes in its protocol and decentralization concept..
Based on these factors it creates demand... and the demand assigns it a value, something which the other party will agree on in exchange for products, services, etc and that's how it goes