to replace its slow-selling 757 and 767
. . . only two of the most successful and widely-utilized aircraft of the last 30 years.
Boeing have just spent a fortune in the last 5 years getting the 757-300 out, as well as making the 767-400 possible.
Recent modest improvements to the 767 have failed to hold off the popular Airbus A330-200.
. . . the 767-400 might as well be called the 777-050. It is very radical compared to previous variants, and is basically a non-round 777.

A bit more than "modest". Really, the 767-400 and the A330-200 are really 6-of-one-half-a-dozen-of-the-other . . . it really depends on the airline's manufacturer preference, the products are very similiar. And the 767 has sold almost 1,000, thus far the 332 has sold less than half. Not really "failing to hold them back".
The 737NG, the 757 and the 767 are so flexible, both up and down, that adding another aircraft to the mix would not really benefit anybody.
If it ain't broke don't fix it, that's all I'm saying.
To me, the 767 is a wonderful, versatile and efficient aircraft, well-loved and established by the airlines, with an excellent safety record behind it. Why would Boeing need to build a new one?
Just unenlightened journalist rubbish if you ask me.
SFly