Exactly!! The current lines are full of passenger trains and especially on the main north south lines. It is getting almost impossible to fit anything else to fit into the current timetable. Moving a large percentage of the current "fast" passenger services onto HS2 will free up space both for more freight and more "stoppers" to feed into HS2.
Even my own line has got to the stage that there is almost no timetable space to fit anything else into the daytime time table, so more freight now is moved at night, meaning there is less time to do maintenance. Result is that lines have to be closed Saturday nights and Sundays.