If Harper keeps playing and progressing the way we hope he does, his first free agent contract will be the richest in baseball history. Boras and several of his assistants will be able to retire and send their kids and grandkids to college with the commission (if they can't already!).
I don't see a scenario where Harper signs an extension before then, but I could be wrong. Who knows what the economics of baseball will look like in 6 years. Or the national economy. Or the Yankees willingness/ability to price everyone else out of the market. I've read rumors that the Steinbrenners are considering selling the team. That could be the biggest wildcard of all. Or another rich team wanting to make a big splash - like the Angels with Pujols.
Best case scenario is the Nats keep winning, selling tickets, selling gear, getting better MASN rights, and building the value of the franchise. So when Stras and Harper hit the market they can compete financially with the big boys.