With other threads on this site, for example, "the implosion of CBS news", and the criticism of MSNBC, it is somewhat ironic that the ABC head is the one who is stepping down. Too much down-sizing seems to be the main motivation, either with a voluntary or forced resignation.
Here is the interesting part of all this. The death of network evening news has been predicted since I was in High School. That was the late 70's, folks. Somehow it has survived. However, I sense this death is probably closer than ever in my lifetime. Let's see how it plays out. Predicting TV listings in the fall of 2012, I would say right now it's about a 40-60 chance these network shows will be on the sked.