Originally Posted by
JFKSFOLAX_friend
enough value to create a sub-fleet?

Nope.
In the case of MCO, not even close.
Originally Posted by
Bear96
But this was an 0800 departure - certainly not coming in from Europe the same day before the EWR-MCO leg.
The aircraft likely RON at MCO for maintenance.
Don't most European flights arrive EWR (and IAD, for that matter) early - mid afternoon and leave early evening? Is that really enough time to reliably be able to schedule a 3-hrs-each-way FL turn?
Yes. Many of the shorter transatlantic flights on 757s arrive by noon, depart for MCO/TPA/MIA/PBI etc. between 12-2pm, arrive around 4pm, turn, arrive EWR around 8pm, then make up flights in the 10pm bank to Europe.
Take, for example, 757-200 ship #3107's day yesterday (actual times):
UA25 SNN 0850 EWR 1051
UA1186 EWR 1239 MIA 1519
UA1186 MIA 1635 EWR 1939
UA114 EWR 2205 LHR 1019+1