E is supposed to replace D and handle the majority of United's international arrivals IIRC. The master plan released last year had them eventually expanding E to replace C. It looks like what was announced yesterday was a revision of that plan.
https://www.mwaa.com/sites/mwaa.com/files/WEBSITE%20-%20Tab%2011.1%20Recommendation%20to%20Approve%20the%20Washington%20Dulles%20International_0.pdf
What I wonder about is whether there is any sort of plan to change how they handle international arrivals. Right now, Customs and Immigration are located in the International Arrivals Building, which is an annex to the main terminal on the baggage claim level. Passengers arriving from foreign destinations that don't have preclearance are segregated and funneled onto the people movers, which take them to the International Arrivals Building for processing. People with connections must then retrieve any checked bags, re-check them if needed, and proceed to the TSA checkpoint before heading back out to one of the midfield concourses (unless they have a rare flight from the "Z" gates in the main terminal). None of the articles about renovation plans have said anything about Customs and Immigration, and that matters because US law requires arriving passengers to clear that process at the first airport they reach (e.g., unlike most countries there is no "airside transit" where you can simply connect, say from Mexico City to Dulles to Heathrow, without clearing immigration controls). Unless they relocate international arrival processing or they come up with some new dedicated way to transport those people to the existing facility, they're still going to need the people movers, even if only for that limited purpose.
Certainly other airports have Customs and Immigration in multiple locations. JFK in New York readily comes to mind, for instance. I have no idea whether that will ever happen at Dulles. It would certainly simplify things for
passengers, but it might make operations more complicated for the airlines in terms of handling checked baggage.