First of all, here is a diagram of the layout of Denver Int'l:
The blue runway is the 16,000ft. under construction right now.
The 16,000ft. runway was part of the original plans for the airport. It's focus was not just to allow heavier, larger planes to fly to Denver, but also to have an even number of north-south parallel runways to allow a proper balance of take-offs and landings.
The reason why there are 2 East-West runways is because a lot of the traffic to Denver comes from the West and also from the East. This will allow faster approaches and decents for those flights that are coming from the East and West. Without them, Planes would need to come from the north of Denver or from the south, and make a sharp turn to land on the north-south runways.
However, the east-west runways are very rarely used for landings. They are almost always used for take-offs. Runway 7/25 is used for take-offs heading towards the west, while the Runway 8/26 is used for take-offs towards the east.
Only once I landed on Runway 7/25. All other times my landings have been on the north-south parallel runways.
Just because you build it, doesn't mean they come. Yeah, I am sure ANA will come once the runway is finished, but they won't be flying 3x daily 744s into that airport. I am sure a daily or 5x weekly 777 would be the main result, with most of the profits coming from United connections.
One of the main reasons why Denver hasn't been very popular with international carriers is because of the economics. If they cannot operate fully loaded aircraft to Denver, then they don't want it.
The 16,000ft. runway will allow airlines to fly 747s and any other widebody aircraft with a full load and this will help attract airlines. No more weight restriction.
Asia service isn't possible non-stop from Denver period at the moment. The new runway will make it possible which will attract airlines.
The fact that 2 Asian carriers have expressed quite a bit of interest BEFORE the runway construction even started is promising. You are right, the 16,000ft. runway will not be a magnet for international carriers. Int'l carriers are not going to say "Oh cool! A 16,000ft. runway in Denver! Lets fly there!"
However it will encourage them as they will be able to fly there planes with a full load.
Europe is not too big of an issue, Asia is the key problem here.
Also, eventually DEN will be congested, and it is now to an extent, and 100 years from now, it won't have all of that open land available. I won't go into details, but it is starting to become that way at DFW, and if it happens at DFW, it will happen at DEN sometime or another.
You are right, DEN will eventually become congested. That is why there is a lot of land which is owned by the city. The full plan calls for a total of 12 runways by as early as 2030 and a total of 6 concourses.
DEN was built in mind for the future. It's currently the best laid out airport in North America according to experts. The way the runways are placed and the distance they are from the gate areas allow for excelent efficiency. When all 5 runways in DEN are used, the airport is a breeze to go through. Almost no waiting time.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran