In response to your comment, I'd like to add the following:
1) You are right concerning the problem Boeing faced when bought MDC. Of course Boeing had to accept the orders MDC had for its MD90 family. Here, obviously, it's included the MD95, now B717. But, everybody must agree on one point: B717 definitly has no future; It's almost 100% sure that Boeing will not, let's say in a 10/15 years period explore an advanced version to it.
2) About competition between 739 and 757: Both airplanes have the same capacity and are created to fit the same market, although the range of the 737NG family member is superior.
3) About competition between 753 and 767:
753 can be used with the same objective as 767. It's easy to see, for instance, in 2 examples:
Condor (German charter) will fly its 753 (and even its 752) in routes to Brazil and Caribe, both
regions in a 10 hour-flight distance from Germany. Normally, an airline operating Boeings would choose to fly 767.
An other example is Icelandair, which is going to use its 753 on Reykjavik-Boston/JFK/Washington services, transatlantic services, which, again, would be normally operated by 767.
And, both 757 and 767 were created simultaneously, in order to provide complementary services. But, if the smaller one (757) became bigger and with an extended range, 757 and 767 became suplimentar, and no more complementar.
4) About 764 and 772: The increased capacity for a 767 operator that the 764 is created to give can easly be replaced by an expansion to a next level: the 772.
5) The 777 has 2 engines. The 747, 4. ETOPS restrictions are applied, and if I could choice to fly a transcontinental flight between 2 airliners - a 2 and a 4 engines - I'd choose immediatly the 4 engines one. So, the 777 doesn't replace perfectly the 747.
6) NorthWest is an Airbus client.
7) The 318, before being an answer to Boeing's 717, is a complement to the well-succeded 320 family.
8) With 777, Boeing doesn't answer the aspiration of companies that want an bigger airliner than 744. Airbus does (A3XX).