The result of doing this would have been Booths/EMR buying them at scrap value as I can't see any operator clamouring for the 373s.
I agree, there might be legal route to compel Eurostar to sell (as was the case with freight locos), but I presume there was little or no pressure on the competition authorities / ORR to bother from potential alternative operators.
With hindsight, the 373s turned out to be the ultimate 'niche' trains, built in far too large a number for a constraint that disappeared only just more than a decade after the tunnel opened and long before the demand assumptions justifying their building came to pass.
Those constraints presumably saddled them with unnecessary complexity and expense that was hard for Eurostar to make a business case for continuing, let alone anyone else taking on for an alternative use. GNER after all replaced their hired NOL sets with more HSTs (20 years or so older), once these became available.
The key constraint on expanding the international market to a multitude of other destinations is the requirement for full security screening ahead of the return trip and I can't see any politician signing off a relaxation of this anytime soon. Avoiding this requires either massive expense at stations to be served, or a long stop at Lille Europe that is an interchange in all but name.
It probably doesn't make any difference to the 373s future use, but I am surprised that they haven't had a look at a commuter flow from Lille / Calais into London. A season ticket plus markedly cheaper housing might have mad this a viable proposition, but with apparently no season ticket available such a flow is presumably now just a handful of people from Lille.
The tax payer could have been saved an awful lot of money had assumptions about Eurostar traffic been more realistic, but maybe they wouldn't have built the tunnel, so perhaps it was for the best (from my point of view). This point is academic now as the 'loss' to tax payers occurred at Eurostar's privatisation. Now in economic terms they are just a 'sunk cost'. Underutilised and fairly new they may have been, but I am not surprised that no other viable proposals for their use have materialised.