20-Year Prospect
There's always a crisis somewhere – usually in the Middle East – which is why you'll have a job until nuclear Armageddon comes along and diplomacy becomes rather beside the point.
However, the diplomats of today work very differently from the deal drivers and kingmakers of yesteryear, thanks to social media. Because there are, for example, 1.28 billion monthly active users of Facebook and 271 million monthly active users of Twitter, it makes sense for diplomats to be on social media, shaping public opinion and providing information about nations and their policies.
Moreover, there's no such thing as a slow news day anymore. Why? Because there are nearly seven billion mobile phone subscriptions in the world, and more than 2.3 billion of those phones have Internet. People can get the information they want when they want it, which means diplomats are constantly facing situations that go from bad to really, really ugly in mere minutes.
So, yes, your job will definitely be around twenty years from now, because a diplomat's work is never done. Just don't Tweet about how tasty the beverages are in a war-torn country, or you could find your diplomatic self out on your diplomatic butt.