Soldato
The capacity freed up is the on the existing West Coast Main Line. By moving the extremely fast traffic away from there, slower commuter and freight trains now have more slots and flexibility down that route. The 125mph trains require a lot of space in signalling terms, and them not being on the existing lines any more gives a lot of opportunity. If it was done at a reduced cost (by lower track speeds) then you make travel slower than it is now. Why on earth would you aim to make the situation worse than it is? Yes, HS2 will "only" shave off a few minutes on the Crewe to London leg but it's not a regression, and to state again, the opportunity for other trains on existing WCML is huge once the 125/140mph trains are gone.
Its not only on the WCML but would have helped the `at maximum capacity` MML as well.