Eurostar/Thalys have a lot of work as their through-ticketing is not only a problem for the connection with sleeper trains, but also with other classical trains and high speed trains, from all other rail operators.
Paying an extra £50 on each through-ticket, and still waiting six hours in the lounge in case of a disruption, is not necessary. A rail operator like Deutsche Bahn is able to offer massive through-ticketing, even with foreign rail operators, without a waiting time of six hours and an extra €50.
Through-ticketing implies an agreement between the rail operators (or a government imposing it for the sake of all).
Most passengers would be very happy with some available folding seats on the Eurostar so they can move on with their journey. Not all seats on the Eurostar Brussels-Lille are filled, and even so, there are classical trains from the parent companies SNCF and SNCB/NMBS from Brussels to Lille. When the segment Lille-London is full, the stranded passengers could change in Lille to another Eurostar (coming from Paris) with seats available.
A through ticketing agreement could include that a through ticket sleeper train to Brussels - Eurostar can only be sold when there is a planned connection time of 90 minutes.
They could even reserve a small amount of tip-up seats in each Eurostar to give a guarantee and peace of mind to all their potential customers that they will arrive comfortably whatever their onward journey,
Not only for through ticketing, but also for other agreements that benefit the passenger like
HOTNAT.
Why not also join the
Agreement on Journey Continuation like the major European rail operators?
Some extra tip-up seats don't cost £50 for each passenger in the Eurostar train with a through ticket. Even if only 100 passengers (of 900) have an onward journey with another train, that would yield £5,000 for each train.
Eurostar can win a lot by collaborating on through-ticketing and by joining the
Agreement on Journey Continuation.
If international train travel would become more robust by through-ticketing and AJC, Eurostar/Thalys would attract far more passengers.