As it is, Jimmy Kimmel is not doing that well at Midnight (ET/PT).
At 11:30, he would get a few more viewers than at Midnight, but probably a smaller share of viewers than he gets now since he'd going head-to-head with both Jay Leno and David Letterman for an entire hour instead of a half-hour.
Plus, he would also lose the lead-in of "Nightline".
Besides, from what I heard, the reason Letterman did not go to ABC in 2002 (and stayed at CBS, reportedely for less money) was that the only way Letterman would move would be if he went on at Midnight, following "Nightline". He did not want to push Ted Koppel and "Nightline" out the door. ABC did not assure him of this, so he said in essence, "Forget it! I'm staying with CBS!".
"Nightline" is perfect counter-programming for the 11:30 P.M. timeslot. Kimmel at 11:30 would not attract as many viewers----or even as many viewers in key demos----as "Nightline".
I hope that Jimmy Kimmel will mention on his show something to the effect of "I'd like to stay in my current Midnight slot. I hope the new 'Nightline' is a success. Watch it. Give the new team a try. Then stay tuned for my show".