Even if Peter Jennings does make a triumphant return to "World News Tonight" (which I hope he will do in the near future), he is 66 years old.
Thus, even if he indeed does return in a few weeks (or a few months), he might not be at the anchor desk for more thananother year or two, even if he is fully cured. I'm sure the thought of retirement has crossed his mind.
I don't know how far along in his treatment he is, but this afternoon (June 30th), an announcement was made which would probably give Jennings a big reason to try to return to the anchor desk, if he is right now almost well enough to get back on the air.
NASA has announced that the next space shuttle launch will be on Wednesday, July 13th (the first manned flight since the Columbia crash). Given the news value, I expect ABC (as well as CBS and NBC) will cover the launch live.
I'm sure Jennings is hoping that his treatments can progress to the point that he can go to Cape Canaveral and anchor his network's coverage of the launch, and then at the end of the mission, the landing. If indeed he should be well enough to anchor those special broadcasts, my guess is that he would be on the air for that special coverage, but not return to "World News Tonight" for a few weeks. This way, he can gradually get back "into the swing of things", doing special-event/breaking news anchoring, but not the fulltime work as anchor/managing editor of "WNT".
As for potential long-term replacements when Jennings retires, I doubt it will be either Charles Gibson and Diane Sawyer for two reasons: (1) ABC may want someone younger who could probably be able to anchor "WNT" for up to two decades (if not longer), and, (2) ABC probably wants to keep Gibson and Sawyer on "Good Morning America", given that the show is now neck-and-neck with NBC's "Today" for the top spot in the morning news ratings. Taking Gibson or Sawyer away from "GMA" might result in a big ratings drop and "Today" again becoming the dominant morning news show.