Maybe off-peak, but in the rush hour I can't see there being enough trains left around the yard that they're able to pull a full 11 car set off line in exchange for another.
What are you talking about? No one said anything about 5 car sets. The R143 in Canarsie (there's no island in the yard's name) does nothing to further your argument either. The R143s operate in 8 car trains, each made up of 4 car sets. When the R143 smashed through that bumper, the only cars that were affected were the rest of the 4 car unit. 8277 was the car that hit the bumper, so that would mean 8277/78/79/80 are out of service, nothing else.