Gold has a history of being a currency as long as civilization so it probably will always be accepted as valuable, and it has actual uses as well, but mainly history. Euro Governments will always accept Euros for taxes, so they will always be useful, and if they stop they will still have a transition plan so you can turn your Euros into something they will accept. At this point if Bitcoin's value isn't going up a lot of people will use interest since honestly that has been its main appeal. And if it isn't technically better than some other cryptocurrencies out there there isn't a reason to think anyone will be using it long term.