20-Year Prospect
The twenty-year prospect for ballet itself is excellent. It's one of the most revered high-cultural art forms, and people will continue to pay top dollar to see top talent move around with beauty and grace for many years to come. If you can become one of the best ballet dancers in the world, you'll have no problem. If you can't, though...
This isn't really a career to be looking twenty years down the road in, even in the best of circumstances. Even for the best of the best, serious and career-threatening injuries happen all the time. There's always newer, younger talent coming up the ranks, and competition never stops being fierce. Even if you make it to the top of the ballet world, it'd be highly unlikely you'd be around for two whole decades.
And yet, passion is passion, so by all means pursue your ballet dreams—just maybe have a backup plan in mind for the almost inevitable move into a different career path at some point.