Boeing stopped making the 757 for the very practical reason that people had eased right back on buying them; not many ordered or produced in its last years.
But the 757 only went out of production a few years ago, they have plenty of life to be around in considerable numbers until the next generation come along.
The parcel shifters are interested in it, sure, but generally in the high time, low priced ones, to strip out and run a limited number of daily hours. If US Airways are whining that they are buying up everything then I suspect they are saying that about the low priced ones.
As soon as the 787 gets going in the US market I suspect you will see a good number of 757 coming on to the market as US carriers switch their fleets around, and this is a major throttling of the 757 secondhand market at present.
The A321NEO appears to have all the necessary to turn current A321 marginal flights like UK-Hurghada, or Canary Islands-Scandinavia, into straightforward operations easily in its capability. I would expect the European holiday operators to be among its early purchasers.
The irony of the end of 757 production was that the assembly space at Renton was reallocated to the anticipated demand for the 737-900ER. But this has been a poor seller, although the smaller sizes continue of course to be very popular. Boeing hasn't had a lot of luck with this biggest 737 - the original 737-900 never got many orders, and after the substantial rework as the 737-900ER it hasn't been that much better. Some who went for it seem to have stepped back, and reordered 737-800s instead. For some reason, Boeing seem to have had this issue with a number of their stretches (eg 757-300, 767-400) in recent times.