MTA's current trains are all in permanent 4/5/6-car sets, and they only ever couple 2 sets together, so there isn't that much flexibility to spare, not nearly as much as with married pairs (train length can basically only be either 1 set or 2 sets, and only a few lines, like G, run 1-set trains). And having a TR-style front end doesn't necessarily preclude sets from being coupled together & running in that configuration, as is the case in Stockholm, or even here on line 5. And the implementation of open gangways changed nothing about the configuration or operational flexibility of each set, since a 5-car R160 is just as much of a fixed set as a 5-car R211, and a full train still consists of 2 R211s coupled together.