As you know I'm sceptical that anything should be done to fix the WCML. I can't fathom how a new line into London Euston is going to open up new opportunities into Southampton. Improvement up here should not be dependent on investment down south.
Capacity up here needs greater work than just downgrading intercity services because everything is transferred onto HS2 from Birmingham.
Southampton to Birmingham currently takes 2:30 at a frequency of 1.5tph.
The journey time from Southampton to Paddington takes 1:35 whilst Euston to Birmingham will take 49 minutes, now that's a total of 2:24. Now although there's a need to change at Old Oak Common that's built in by including the journey into/out of Central London.
That's probably not going to attract many people over the direct service, but it does give you another option of there's problems with XC.
From Reading you save the problems of the Basingstoke/Reading section and so you'd have the choice of 3tph from OOC to Birmingham over the 2tph using the XC services. This would have a journey time of ~1:20 Vs the existing ~1:30.
As such there's no need for any additional investment in the South for people to benefit from more services.
As such it could lead to a switch off passengers from XC to HS2, freeing up space for passengers who remain on the XC services.
However with the opening of the Southern Approach to Heathrow there could be a better journey time saving from Southampton with s 49 minutes Southampton/Woking, leaving ~50 minutes to get from Woking to OOC when the cited times are likely to be ~40 minutes.
I've pointed out before that Piccadilly could benefit from extra capacity which could be created by the removal of the long distance trains from the current services.