I think cryptocurrencies will coexist with normal currencies (they kind of already do), but it's hard to tell. If bitcoin was to be used effectively in the future, I guess transactions would have to happen off chain, as if you have your cryptocurrencies in a bank, similar to how coinbase users can theoretically send bitcoin fee-less to each because the bitcoin doesn't actually have to change wallet. The whole trustlessness point of cryptocurrencies will kind of disappear, though, but maybe that's what it takes.