If you want to improve on three-dimensional modelling of physical systems (such as that used by the Havok physics engine in many popular current video games), differential geometry is going to be *very* useful. As with other applications, it's a niche market. This is the way of the modern world, very few skills have applications to all walks of life.
Of course, once created, such an application can often be used by the end-user with little or no training in the skills used to make it (by analogy, one does not need to be a machinist or automotive engineer in order to drive a car).
Many scientific and mathematical concepts are like this: only truly useful (in the sense of getting paid for knowing them) in small specialized settings. Of course, teaching others being the "obvious" application.