20-Year Prospect
As long as there is government funding for wildlife conservation, there will be Game Warden positions. This is less of a sure bet than you might think. The National Park Service has only been a thing since 1916 (source), and certain organizations and political parties have been looking for a way to dismantle it ever since.
When fiscal belts tighten, environmentally-related expenses are often the first thing to be cut, and that means fewer jobs. The only way to increase conservation funding is to get people to focus on environmental concerns (source). Will it happen in the next twenty years? Only time will tell...but on the plus side, your training will allow you to work as a regular everyday officer of the law as well. If there is ever downsizing, you can apply for jobs in numerous other departments.