Wouldn't this also be slower than the WCML and therefore not move any of the custom off the existing services on that line?
Yes it would be slower. You may get a few switch, but mostly not. In reality all that it will do is attract new people to those services (no bad thing) but it's not going to improve capacity on the WCML.
Having done that, what about the MML & ECML, what scheme do you build for them? What about the capacity improvements for XC from the removal of the city to city travelers? Now add the costs up, is it close to HS2's costs and what improvements have you made to capacity?
Probably closer to, if not more expensive, with a lot less capacity added.