20-Year Prospect
What you're selling may change in the next twenty years (anyone need a hovercraft?), but the odds of sales careers vanishing are zero. Some sales, such as retail, have drastically changed as online shopping has become more popular. Still, there are things almost no one would buy online and have shipped directly to them without seeing them first.
But, for argument's sake, let's say the future brings a sizable number of people who want to buy big ticket items and have them delivered. They'll still need sales people, and their supervisors, to explain the product and make sure that the customers are happy with it after delivery.
Besides, in twenty years, people will still be needed to supervise the sale of Girl Scout Cookies and hand out award pins. Some things are truly timeless.