you just defined 'currency'. basically, your issue with bitcoin is that it can't be exchanged for goods and services.

this is quite the chicken and egg problem. but basically when people 'believe' it's a currency, they will accept it as one - thereby justifying that belief since it will be true due to all the other people accepting it.

dollars are only a worldwide currency because people and institutions think it's one - and if it were to stop being treated as currency it would stop being accepted as one. totally tautological - but there's currency for you.