If the Greeks stop repaying them, that forever money spigot gets turned off. But if the Greeks keep paying it off, it's the best deal a banker can have. It's very Mafia-like to loan someone an amount they can't pay back and thus own them, but have to keep making payments on that forever-loan.
And you really think the Greeks are going to keep paying it? With what?
Go ahead and give a homeless some money and see how much you’re going to get back.
What the germans and others are getting back is their own money that they lent the greeks to repay their loan.
In your case it would be like the bum you lent money to stops repaying you (because he doesn’t have any money) you lend him more money which he uses to repay you with.
Smart move grania.