ROI to US, regional UK to US and a few European to US routes need a B757 as nothing else will do the job as well. However that's a niche market in double figures, Boeing would need substantial *MASS* market requirement to build a new B757 replacement.
It's not enough to justify the capital investment. Look at the poor sales of the B747SP, B767-400 and the passenger B747-8i.
Last edited by Skipness One Echo; 26th Mar 2013 at 17:22.