what are the odds of ABS brakes failing in a 2003 Volkswagen Jetta?

The odds are very slim.  The system consists of 3 main sets of
components: individual wheel speed sensors, mechanical system, and
electrical system. All three are connected together via the On Board
Diagnostics system and they all communicate with each other. If one of
the aforementioned components fails then the system will instantly
recognize this and will display the check engine light on the dash of
the car. Of those components, the wheel speed sensor is the most prone
to failure but the number of cars affected is very slim and the driver
is made aware of the problem as soon as it is detected.

There is nothing that will happen to the ABS system without you knowing.