We used Eurostar for Holidays pre covid. From what we saw all the intermediate stops with the possible exception of Lille generate relativley small numbers of passengers. So as others have said if Eurostar did collapse then scaling it back to just London - Paris and London - Brussels would probably result in something which could be run and be profitable. London - Lille could also work, there are good onward connections to Paris and Brussels, as well a connections to other parts of France. Lille could be a good interconnection, there are TGV services to the South and West of France, but Eurostar services never seemed to connect with them, and ticketing was a nightmare. If you were going to reduce Eurostar then London - Lille would work in terms of connections, but Lille Europe as it stands would not be able handle either the passenger flow, security etc or the terminating rail services, and after a collapse any new operator is not going to want to invest millions in new infrastructure.
Eurostar is also attractive to those who live a long way from any airport, our nearest airports are Humberside, Leeds and Teesside, all 3 are about a 1h 30m drive, factor in another 30mins to park and get in the airport, then the check in time, which with low cost flights tend to be longer, and train starts to look attractive. The flight options are also quite limited, which then means looking at London or Manchester, once you come into London you end up at Kings X anyway so may as well catch the train onwards as its just a walk across the road. The problem with rail is the ticketing, it will discourage travellers with significant journeys at either end as sorting it out requires a 'degree' in UK and European rail ticketing, its not so much about the cost, its about trying to join it all up. So if Eurostar or a new operator wanted to increase business with minimal outlay, sort the ticketing out, and make the punters aware.
Even post brexit I dont think the government would want cross channel trains to cease completely so I think a deal would be cobbled together behind the scenes to keep at least a basic service running.