The existing line is used for passengers and freight, slow trains, fast(er) trains, freight trains etc all sharing the same line. HS2 is primarily to get most of the passengers off the existing line and leave that for freight and stopping services etc. The sad thing is that early on it became known that the new line would be N minutes quicker than the existing one for passengers. Hey presto the media and general ignorencia focus only on that. Hence the general public, like the questioner here think that the time difference is the reason for doing it.