Here's what I think of it.
The doesn't need service to Brooklyn. In 2005, I think, the ran between Bowling Green and Bronx (I'm not gonna try to spell it) except rush hours to Brooklyn. Then it was increased to run to Brooklyn in middays. This is pretty much proving that the extension isn't needed, and by the time evening hits, most people have gone home already.
About the , you will be causing congestion between Canal Street and Bowling Green to all , , and trains. and run at 2 minutes, and so does the . Put them together and you would have a conga line of delayed trains coming into Bowling Green every minute. And those 2 select trains still are gonna create delays, because as I said earlier, the and trains run at about 2 minute intervals, and squishing a in means you will have to get the to terminate, and leave immediately. It will also delay and trains because they run at about 3 minute intervals. Holding the is required for the action, and that hold will delay / trains.