A few things might contribute to make it possible:
- There are speed limits, a lot of them. There are speed limits around cities, before interchanges, in areas with a lot of traffic or many curves, etc. Increasingly, you will see road sections with dynamic speed limits being turned on or off depending on the conditions. So even in Germany there is an awareness that speeding can be dangerous, people do not speed to the same extent everywhere.
- You're still responsible for your speed. There are no special safety features, no broader curves or anything like that. If anything, Germany has been underinvesting in its infrastructure and road surface tends to be worse than in neighbouring countries. But drivers are aware that some cars will be very very fast, which might alleviate the danger if you compare it to illegal speeding in other places.
But the key is that nobody knows what the effect on safety is and there are many reasons to think it does have a negative effect overall. All you can say is that it's not dramatically worse to the point that Germany would be much more dangerous than other European countries.
Historically, the number of deaths on the road (appropriately normalised) tended to be lower than in other countries, which specialists explained mostly through the higher traffic density (more cars on the road means more traffic jams and therefore less deadly accidents). Now, it's in the middle of the pack, not much more dangerous than other European countries and certainly safer than, say, North Africa, but not especially safe either.
The thing is that it's a game of numbers, cars don't break down when you pass 140 km/h in another country and in fact you can find many countries in the world with worse road infrastructure and lax rules (or strict rules that aren't enforced). So everything is “possible”, it will just make road slightly safer or slightly more dangerous.
And, ultimately, the issue is highly politicised and, AFAIK, no separate statistics are available on non-limited stretches of motorway (which would be necessary to make a head-to-head comparison or try to model the effect of speed limits).