Skipness' & Theloudone.
The A321 cannot do anything near what a 757 can do, and a few 'tweeks' to an existing Airbus design will not replace a fantasticly capable aircraft.
The unique capabilities of the 757 are derived from its conception. It was designed as a B727 replacement, hence the phenomenal performance.
And as a comparison with the A321, some years ago I worked for an airline that was called in to provide a couple of 757's for a winter contract based in the Canary Islands. The routes operated where between the Canaries & the Scandanavian airfields at the top end of the Baltic, iirc, Umea & Lulea were 2 that I operated to. These were 6-7000' contaminated strips with 220 seat config'd aircraft. Our 757's easily carried the load non-stop for the 6.5 hr+ flight to Tenerife and/or Las Palmas, whereas the customer's (now demised Spanish scheduled carrier) much vaunted, brand-spanking new A321's couldn't match the 757's abilities. In fact their A321's had to tech stop! Once on the leg northbound & TWICE on the return leg southbound!!
Another time, I was based in Dublin & one of our regular routes was To/from Heraklion. Not a problem on the 757 but our base at Belfast used an A320/321 on the route & every time we were delayed it was because the Belfast aircraft couldn't get airborne for performance reasons so we had to carry their offloaded luggage. The A320/321 family were designed as 2-class hub-feeders for major scheduled carriers NOT high density 'coaches' for charter carriers, hence their sometimes woeful performance.
The 757 was conceived to operate 2-class 'shuttles' from Hot & High, Noise Sensitive, or Performance Challenging Airfields & long, thin 'pioneer' routes TransContinental and/or TransOceanic routes such as Europe to US East Coast, where it complements its stable mate the 767. The combination pioneered common type ratings & excellent flexibility of capacity ranging from 180 seat 757 to the 300+ 767-400, for the same lucrative routes flown by the same crews.
Airbus will NEVER change from their strategies of a Hub & Spoke mentality (ie. A318/319/320/321 feeding A330/340/350/380) just to steal Boeing's thunder. And Boeing as the philosophically 'Point to Point' specialists already know that they can only grow their 737 family so much before they have to re-visit the drawing board... or an existing design. IcelandAir are only reluctantly accepting the B737X. They would've liked new-build 757's!!
I was fortunate enough to be on a Customer Factory Tour at Everett in 2008 being shown the 787 prototypes & production line & I asked one of the senior sales officials who was showing us around if Boeing were ever going to re-open the 757 production line because of the capacity gap. He replied with a smile & said that if Boeing had a dollar for every 757 customer who had asked that then it would be financially viable to do so but everything was being done around the 787, however he did say that the tooling & jigs from the 757 were still in storage & that knowing Boeing as he did, there was probably someone in an office at Renton looking at adapting newly available technologies to existing designs.
An A321 that can seat 228 pax & fly non-stop Europe to JFK, EWR, BOS or IAD? I doubt it. We'd have seen it already.