Shorter journey times, less overcrowding and more services arent tangible?
If thats the case, what on earth is? Providing more capacity while reducing journey times are the most compelling reasons for investing in any transport infrastructure.
The GCR is not suitable for any kind of HSL - while it was built to a generous loading gauge like the GWR it certainly wasnt to any 'continental' standard which at the time simply didnt exist; even if it was, any new route would have to straighten out, divert round or tunnel over so many areas that it would be cheaper, easier and less controversial to build an entirely new route from scratch.... and if you did that, there's a far better business case for mirroring the WCML, not the MML. Which gives you HS2.
Chris