This is all based on the success of the evolutions of the A321. As TDRacer says, can Boeing really compete by evolving a very old design (757) ? There is generally a huge love of the 757 because it is an aesthetically pleasing design (especially with winglets), the love pilots have for it due performance, and the unintended niche in TATL flying it found in later life. How big is this market? Can it sustain 2 competing airframes when we consider crew currency? IE the crew can fly a Boeing equivalent of the A320 on a 2 hour sector today, then an 8 hour sector with an A321XLR equivalent overnight tomorrow on the same rating? Is it actually worth Boeing's expense to divide this market with Airbus? In a similar fashion to the B748 vs A388 argument....have both manufacturers lost...or have Boeing lost "less" because the B748 has the freighter variant.
I think we're mixing possibilities here. The 757 achieved what it did because of powerful engines, big wings and double bogey gear to get good runway performance and range at the expense of fuel burn due those thrusty engines at the expense of weight. In the modern world, are you better with two different types to achieve same mission or one type compromised in one to accommodate the other? No doubt a new bespoke design in the niche would knock Airbus' socks off, is the market in that niche big enough to justify the expense?.....