friend in fleet planning said there are no plans to replace the 737/744/757/767/ fleet anytime soon. He said most of those birds are owned... so maybe in 20 years, you'll see a different makeup. He said everything so fare is chit-chat.
United already said it won't go w/ the A321 for operational reasons... it's not a short field performer like the 757.
Only thing on the books right now is dumping the 767-200s for -300s and UA is waiting to see how CO/NW/TZ are doing w/ the 757-300 before ordering a few.