Don't loop terminals somewhat limit capacity because there is a policy to get everyone off the train? I've seen considerable backups on the 6 at Brooklyn Bridge.
You're confusing loop terminal with loop track. The loop terminal was City Hall. Brooklyn Bridge is a relay terminal. So is Bowling Green. South Ferry is a loop terminal.
Last stop. Everybody off.
That's a recent TA operational procedure at relay terminals. Its effect has been to further reduce system capacity without much of a benefit for security - its supposed purpose.