Good point but enhancing an existing line to improve capacity and speed is far more problematic than building a new line on a greenfield site. I think they realised that after comparing the success of HS1 (Channel tunnel to London) when compared with the West Coast main line upgrade that was taking place at the same time.
- There is a finite limit to the number of trains you can run down any stretch of track. Once you reach that limit (which is quite close on existing track) You have limited options to increase capacity:-
> Make the trains/platforms longer. Good in theory, but requires major changes to existing infrastructure. (Demolition of existing buildings in town centres) Changes in track layout, particularly at terminus stations. Changes in signalling (for longer trains).
> Double decker trains. This requires a change in the loading gauge of the lines. A particular problem in the UK that has a smaller existing track gauge than Europe. This is why double decker trains are widespread in Europe and non-existent in the UK: there simply isn't the room for them. Changing the gauge basically means rebuilding the entire railway, with all the disruption that brings. (i.e. rebuild bridges, overhead lines, all track-side structures, track alignment, platforms....)
Building an entirely new line brings you all of the benefits of longer platforms, double decker trains, and a much higher speed. All without causing any significant disruption to existing lines. It's cheaper in the long run. And it provides a much bigger increase in total capacity and resilience for the money.