I'm not at all convinced that this is true.
Transport demand is one of the most price sensitive things I can imagine in existance.
Especially when HS2 puts a huge number of people very close to major load centres (London etc).
If cheap, rapid transport is available, people will inevitably build lifestyles that use it - this has happened throughout human history.
I think a model that drives demand sky high with low fares, primarily using crowding to control passenger numbers, could work and deliver relatively low subsidies.
It's not cheapness that is key, it is *Affordability* that is key. Cheap and Afforadable are not necessarily the same thing. Once the price is at a level that people in general feel is reasonable for the journey, then there is no benefit in reducing the fare below this.