20-Year Prospect
While basketball continues to grow in popularity, football gets the ratings, and soccer tries to graduate from the B-team of American sports, there is no greater pastime than the game your great-grandparents called "stickball." With literally billions invested in its long-term existence, baseball is here to stay.
Unfortunately, so is the amount of umpires that MLB employs. While an eight percent growth rate is foreseen, that's mostly at the high school and college level (source). It won't get any easier to break into the Major Leagues.
Baseball is a hallowed American tradition, like apple pies and cute puppies. This works in your favor: technology won't change the rules of baseball. It also won't change how you enforce those rules, since you'll have to call 'em like you see 'em—not how you see 'em later on instant replay.
Until Google Glass becomes cheap enough that people feel comfortable putting it in the same space as ninety-mile-per-hour fastballs, you'll be relying on your eyeballs to tell you what just happened. You know...the old-fashioned way.