O&D traffic at IAD is growing exponentially.
Remember, IAD was one of the nation's fastest growing airports in the late 1990s. This was attributable to the explosion of tech-industry growth around the N. Virginia corridor, which incidentally is the wealthiest area per capita in the nation.
Once the economy rebounds, Dulles offers the most revenue growth potential of any single hub.
With these flights, UA will have 3 GRU flights again (ORD, IAD, MIA), but only 2 out of the former 3 EZE flights (IAD, MIA).
However, I think JFK-EZE is returning for the winter season only.