You can only run so many trains per hour on each line. It's true you can increase capacity on some routes by lengthening trains. But on intercity routes most trains are already 11 carriages long and there is limited scope for extra trains to be run during the day.
Basically we need to build now so we have enough capacity from 2030 onwards.
Even if we didn't use high speed trains we'd still need to build additional capacity on the WCML. I suppose upgrading WCML to use double-decker might be an option, but that would involve rebuilding I don't know how many bridges and tunnels on route and years of disruption on the existing lines. It wouldn't be cheap either...
Nothing to do with railways is cheap. I think the recent electrification projects have averaged around £6m per kilometre.
A recent project to double up a single line track near where I live cost £45m for about 5 miles of track and went nearly 50% over budget, despite it being both straight and on the flat.
Why don't they utilise the motorway network to reduce costs?
Build the line above or next to the motorway as we already own the land.