20-Year Prospect
Considering field offices are being shut down and hiring freezes are in place, it's unclear what the future will bring for air marshals (source). With more armed airline pilots out there, there might be less of a call for air marshals on planes (source). Or perhaps the powers that be will decide it's better to have two sets of eyes—one at the controls, and one in the back—keeping a lookout for trouble. In other words, when it comes to working as an air marshal, time will tell.
That said, the agency has been around for a very long time and we do need someone policing the skies. The job may just change in nature or in training.
Should something like 9/11 happen again, it's likely there will be another rush to hire air marshals. That's great for your career prospects, but lousy in every other possible sense since terrorist attacks are, like, minus infinity on the fun scale. All in all it might be better for air marshals to fade gracefully out of the picture; if we don't need them, it'll be because the world of air travel is considered safe enough that we can tell our faithful air marshals to take the day off.