20-Year Prospect
We're fairly positive gardeners will still be around twenty years from now—the grass ain't gonna cut itself, and people don't seem to be getting tired of having plants around. In fact the opposite is true. We're seeing more and more efforts to preserve plant life all around us these days.
That said, gardening is probably going to change a bit with the times. Gardening has come a long way since the pre-motorized days, and we see no reason the gig will stop changing in the future.
Technological advances in gardening are already kicking it into the digital age. In 2013, the U.S. Forest Service warned that trees in urban areas were disappearing at an alarming rate.
This problem led to one Forest Service researcher creating the i-Tree app, which provides lots of information to city and community leaders about trees and managing forests. This helps said leaders plan out the best ways to incorporate trees into their city planning and designs for the future, which includes advice from gardeners just like you (would like to be).
Advising city leaders on planting more shrubs, flowers, and trees is not only going to create a better environment for inhabitants, it will also provide job security for you. What a lovely coincidence.