One of Boeings' strengths is that they provide what the customer asks for. If the customers were asking for a direct 757 replacement, and were willing to pay for it, Boeing would build it.
The 757 is my favorite airliner to fly, and I believe the most capable and flexible. But airlines are not asking for another one. For most of its career, and the missions it flew, it carried around a lot of unused capability (weight and cost). Most 757 flights are far short of its' max range, and mostly off of long runways. The US market in the 80's and 90's was probably its highest, best, use. And that is what it did.
It is my favorite airliner, and I just slammed it. It is not being replaced currently, because it does not fit what the market wants.
I don't love it any less.