A conventional train and close headways are not an either-or proposition. Freight trains can (and do in places) operate under computerized moving block systems that allow for headway as short as relative braking distance. Automatic train operation with moving blocks is a relatively mature technology available from a couple of major vendors.

There are certainly advantages to single-unit cars that avoid the need for shunting, but the maintenance cost on these will be tremendously higher on a per-unit basis. Hard to say that it's a clear win.