Presumably Eurostar have a lot of yield data to decide their pricing strategy - are you suggesting they should accept lower profits?
Well Eurostar's data is only part of the story
The Government does indirectly subsidise EUrostar, as it ate a substantial loss on the sale of the concession to the current HS1 operator.
The HS1 operator also does not optimise for societal benefit, it optimises for its own financial gain.
In essence, HS1 is currently operated in a way that is optimal for the private shareholders but is unlikely to be so for society as a whole.
Then again, the optimum-for-society way to operate HS1 is probably not to fill it with Eurostars, it would likely be to stack it full of domestic trainsets.
But this wider problem is found throughout the UK rail industry.
The farebox is now only a small portion of the cost of operating the railway, consequently we would expect the "optimisation of fairbox" and "optimisation of societal benefits per pound spent" options to become increasingly divergent.