I remember Eastern Airlines CEO saying after one year of 757 operations that its a great aircraft and that Boeing will end up selling more 757's than 727's, unfortunately he missed by almost 800 units ( 1831 727's - 1050 757's ), probably one of the main reasons of its demise is Boeing's own 738/9
It would be quite easy to label the 757 a disappointment in comparison to the 727 (recall that it was initially conceived as the 727-300), but that's not the case if you consider the whole picture.
The 757/767 duo were higher capacity replacements for the 727 and 707. Actually, it was a smart move to go bigger, since the 737 was thus given more room to grow (initially to the capacity of the 727-100, then 727-200 and beyond).
If you consider that the "bottom half" of the 727 market was divvied among the 737 (-300 and -400 initially), MD
-80 and A320, the numbers for the 757 are quite healthy. Also, Boeing was able to offer a single product which could fulfill the missions of both a 727 and 707.
Amazingly enough, I haven't ridden on one yet; however, I'm looking forward to my first opportunity next month on a UA LAX
roundtrip. We're also "due" to go to Hawaii soon, so there's a good chance of getting onboard another one. Of course, they'll be around for years to come...