20-Year Prospect
In a perfect world there would be no need for the services of a private detective. People wouldn't sue each other in court, bills would be paid on time with no complaints, and no one would ever cheat on their spouses. Sadly, that's nowhere near the world we live in.
As long as people have secrets and want to hide those secrets, the work of a private investigator will always be with us—and thanks to new technology, it's becoming easier to get to those secrets every single day.
From GPS locators to advanced laboratory equipment to good old-fashioned social media sleuthing, technology is changing investigation in remarkable ways. Pretty soon you won't be able to sneeze without the entire world knowing about it.
It gets even crazier when you consider that drones are becoming part of the gig (source). Now an investigator can spy on targets from a safe distance with a flying object that most people might think is a bird.