20-Year Prospect
There shouldn't be too much to worry about in this career, longevity-wise. Movies, TV shows, theatrical productions, and most entertainment events all require lighting designers.
Unless we evolve into subterranean mole people who only listen to our entertainment via interconnected mole radios, lighting designers will be a necessary part of the entertainment world for many years to come. (And even if we did become mole people, it would take way longer than two decades.)
The job is also artistic enough that it can't be replaced by emerging technologies. Robots are going to be stuck holding the lights, not deciding where to put them. On top of that, there are more avenues than ever (thanks to streaming services) to get creative visual projects broadcast to the masses. You should be all set, on set, and setting up lights for years to come. Not a bad setup, eh?