Of Course re-opening the GCR is not the 'magic solution' there needs lots more to be done, but at least opening the old routes accesses more places that do not have a railway now, and that is my point, and they will take you to/from places that people do want to go to
I agree the London end needs to be addressed, Marylebone has a little extra capacity available, Euston does need to be re-built anyway, the old part of St Pancras could be worked upon, and it's a pity Broad Street was demolished and built over, so maybe we could use another London terminus, but what is HS2 planning?
Opening up old lines will add many more paths, and you will not need as many paths if you relieve the capacity of the mainlines by re-opening middle- distance 'relief' routes, however I do not think of them as relief routes, they are places where many people actually end up at, when they have completed their journey from the mainlines by bus or taxi
And the number of passengers in future will not rise infinitely, the British Isles can only home so many people, what that number is I don't know, but not infinite.
My Point is why open a new route with no stations other than the places that have already got several stations, that just gets you to these places many people already go a bit faster, and not as fast as the airlines already do, when lots of towns are crying out for railway connection the government plans to spend billions of our money on building a railway that does not connect any new places, opening old routes will also cost billions, possibly less than HS2, and its at least where people actually do want to go to/from and it would relieve the current mainlines.
It is always worth repeating the same old story if people have forgotten it, you can learn from History