I can't think of a single route that the 757 does that the 738 cannot do other than the odd Xmas USA special or ad hoc MOD charter the 757 needs to be full to make any sense or money against the 738 and this is at a time when apart from half term weekends even the 733 are far from full. It looks like the second one will be Manchester bound as well for crewing reasons
EWR - MAN, BFS, BRS, NCL, BHX, GLA, EDI, DUB
JFK - MAN, DUB
PHL - DUB
That's just UK/ROI from the top of my head for 3 of the US carriers.
There will be dozens more across the globe.
Also - if you can get a fully loaded 737 to Berlin from NYC - do get in touch with Boeing. They'll want to hear from you...
If you're on about European/medium haul routes, you may be right. I don't know how a 737 would do from Egypt/Cape Verde Islands to the UK in the height of summer though...
But do remember how LCCs work, certainly in LS's case. The fares start low and move upwards. Therefore the extra 46 seats may be (most likely) will be sold at far above average/break even fares therefore contributing to offsetting the extra costs involved with a 757 as opposed to a 737.
Just my opinion anyway.