20-Year Prospect
As long as there have been celebrities, there have been minions catering to every need of those celebrities, because—let's face it—being famous is like super-hardsies. And, since celebrities will be around as long as money and cameras exist, we'd bet their personal assistants will be as well.
When the Brangelina of the future and their spawn take a vacation to Australia, they'll still need someone to plan the trip for them; clicking around for tickets and hotel reservations on the internet is for the unwashed masses.
The celebrities of the future will still not know their own Space Mail addresses or feel like ordering their own Starbucks from the replicator. Calendar apps may get beamed directly into their brains, but they'll still need someone to fill the calendar in for them. Unless robots are developed that can pick up the dry cleaning and be on the receiving end of an angry tirade, expect the personal assistant to still be in high demand.