If HS2 is strictly about relieving capacity on the southern section of the WCML, then I'd think that looking towards the Chiltern Main Line would have been more cost effective; compulsory purchase of adjacent homes so Marylebone can gain more platforms and reinstating the other running tunnels at the throat of the station as well as 4 tracking throughout would be more cost effective I'd think. Plus it would allow other services like the Met to Amersham to come under Chiltern's umbrella, and therefore badly needed track work on LUL's lines (which would pass to Network Rail, or Chiltern directly) to be undertaken, and electrification to occur. Aylesbury should not be an hour away from London considering the distance. These improvements would allow other services to run; and even if you only electrified the suburban sections you could have cascaded HSTs running longer distance services (VTEC have overhauled theirs, the same could be done to GWR units and would make the upgrade more cost effective)*. This would allow a far more efficient railway overall to be run (added diversion opportunities too), and since Brum-London is effectively a London focussed scheme I doubt many would miss it, and would improve various services. Furthermore I think that there's potential for a
faster service on the CML than there is on the WCML anyway; I think 80 minutes is achievable there without in-cab signalling and 140mph trains (IC225s?) With them you might well crack 1h 15 - which is only a bit slower than London to Ipswich!
Furthermore it would benefit commuters in two ways:
The seats would be cheaper as those works would certainly not need a budget the size of HS2's
It would spare them from the horrors of Euston itself and also for those services still running there, the horrors of trying to fix it.
And allow them to experience the charm of Chiltern's terminus.
And it means that one would not have to use the awkwardly placed Curzon Street.
*However, I think that HS2 phase 2 should be built, as this actually benefits the Northern power house. And yes, it also puts more people in range of London, but that's not relevent. If all the stock was classic compatible you could run through services directly there. The only disbenefit is that you'd have to check which terminus to use. It'd also mean that HSTs would not have to be used (which would make me sad personally but environmentally and for most passengers it would be a positive). Spare money could be set aside for HS3 if it's allocated for the Northern Powerhouse scheme, extending the Central line to Denham so Chiltern can ommit calls there, or improving accessibility and future proofing stations.
Sorry if I excessively crayoned/repeated a previously stated idea. It just popped into my head.