20-Year Prospect
In twenty years, people will still have teeth. These teeth will get tartar, plaque, and decay. They will need to be cleaned by a hygienist.
So the job will still be there. In fact, a 33% growth rate is expected between 2012 and 2022 (source).
The tools, however, will probably be more sophisticated. They'll likely get faster and more ergonomically friendly for the hygienist. The revolution has already begun with new technology that is used in other forms of medicine, and it's only a matter of years before it starts to cross into dental hygiene usage (source).
Everything's better with lasers.