Well, it's not isolated in the North American sense, but then again, Frankfurt is within easy ICE reach of all those cities too.
They can build the airport right, and I agree that you might as well make provisions for future growth. Even so, I think it's unlikely that that growth will materialize.
Berlin is Germany's most cosmopolitan and exciting city and the one that I'd want to live in the most. Still, despite the massive amount of infrastructure investment from the German federal government, it has failed to become a lightning rod for economic growth. This is true for all of the former East Germany, as a whole.
On these urban discussion forums, we believe more firmly than others that investing in new infrastructure will be a panacea to the economic and social problems that plague our cities. "If you build it, they will come" seems to be a very fervent mantra of ours. Berlin is a sobering reminder that this is not necessarily true.