Well, there are plenty of examples of doing meduim-to-long haul with narrowbodies:
OS doing VIE-DXB on 738, that's 5:30
and for a while they been doing VIE-CMB via DXB on the same metal, that's close to 10 hrs plus techstop
BA doing LHR-THR on 321, that's 6:00
BA doing LHR-ADD on 321 with one stop, close to 9 hrs plus techstop
GC doing LGW-BJL on 738, that's 6:00 as well
CO flying to Europe on 757s
When it comes to the charters, hold your breath...
One Scandinavian operator used MD83 to fly from Scandinavia to Carribean, there are plenty of 757s deployed on Europe to Far East routes via DXB or MCT or somewhere else. 757s also not rare on Europe to Florida and even Carribean runs...
It is the market who dictates which equipment is suitable and which is not.