Both account keeping and converting one currency into another are well understood problems for regulators.
How do you figure? Germany, like any other developed country, can simply go to the delta: merchants who want to stay in business. Want your business license? Better comply with currency laws.
Bitcoin is too useful to be squashed by mere government regulation. If a few more countries employ these draconian measures, some cash-starved government will welcome Bitcoin in the future.
If regulators ("the body of Leviathan and the head of a social worker", to lift from Joseph Stromberg) were smart, they'd advocate easy exchange rules and impose a small tax on each transaction on the exchange.
Over the course of generations, I tend to have more faith in markets than in governments. That's why I think regulators will fail in their efforts in the long run.
So the state has a lot of tools.
That really annoys regulatory mindsets.
Bafin does not seem to care if you use btc to trade with your friends, but once € are involved they make the rules. Don't know if that angers you, but I think they have a case.
Not so easy with Bitcoin