It doesn't actually do that. It replaces one route to London with another.
People won't be travelling to Euston on the same lines as they currently are. You won't be able to get a fast train from New St to Euston any more, that franchise will not exist, which is why the franchise for the NW Mainline had a condition of running High Speed networks
There is no extra route, there is no extra focus. The express train to London will only use one route.
The current routes will be used by more local services and freight. Slower trains require shorter stopping distances and that massively increases capacity on the lines. Separating the express trains from the slow and semi-fast trains adds huge capacity