Quant trading algorithms, like most stochastic trading approaches, lose their effectiveness over time as the market internalizes them and as knowledge dissipates naturally (not necessarily through leaks). The best of the best individual quant trading algorithms have a half-life that does not exceed one month. And we're talking exceptional here. Also, at any given time many, many algorithms will be running simultaneously; complementing, overriding eachother as necessary.

So there's a lot of churn overall. The code that someone picks up in this way contains a single snapshot of this transient trading strategy, not particularly useful to run on their own. You're right that, what really sucks for Goldman is that someone could find some basic patterns inherent to their trading platform and trade "against" Goldman.

The catch is that this sort of maneuvering happens all the time. Sure it's going to be painful in the near term for the Goldman quant guys, but they don't need to start from scratch necessarily.