Interesting concept. I agree with the writers the A321 looks set to take over the US based 757 fleet/ transcon. AA/US has 300 A321s on order/ in service. Jetblue, DL ordered fleets that will soon enter service. UA no doubt will follow, for their Houston routes southwards. 737-900ERs don't do them.
Problem for upgrading the 757s would be their OEW but even more I guess their age/ hours / cycles and old systems. I have the impression the US operators are really wearing out their 757s lacking a real replacement..