20-Year Prospect
People will always need furniture. The styles may change, but the need won't.
Gadgets and technology could be seamlessly incorporated with furniture design.
The whole process will no doubt become more automated, though. Even today, people are able to buy furniture online. That's not a trend that's going to decrease anytime soon.
However, there's nothing quite like actually seeing and touching the furniture you are looking to buy. For that reason, the job isn't likely to go out of style completely. Especially not in the next twenty years.
You probably won't want to stick with this sort of work for twenty years anyway. Most people either transition into management, or are just doing this while they progress towards something else.