Interestingly AA (American) are now saying the 757 seat mile costs are "a problem" (presumably versus their 737-800)
It's unique performance was very much a serendipitous outcome of the times, and available technology (ended up being "overwinged" and "overengined"). ISTR a study that showed a huge majority of the flights it operates though (95%+ or something) don't need 757 performance. Which is why it has been replaced by B737-900ER. The 787 is a 767 replacement, not 757. Operators are just not interested in paying extra for capability they don't need