It's a small problem given there's only 1tp2h from Eurostar and lots of other options as @dutchflyer says. Certainly smooths out the border/customs issues at Brussels, because when you could do the trip in the return direction AIUI the French/Belgians were insistent that full passport checks would not be made on internal Schengen passengers, while the UK Home Office was paranoid about people trying to sneak through the border by buying a ticket for Lille and then not getting off. Demand is heavy enough for Brussels-London that Eurostar loses very little revenue from it.
Do alot of Belgium people visit London? I found it very busy. I know Paris is more popular hence why National express want it set up a rival. I was at Rotterdam and Eurostar looked quiet. I notice most of the E300’s run to Paris rather than Brussels now. That’s why I asked when they plan to replace these trains. Yet to see any E300 trains get the new logos?
There's a number of different flows between Brussels and London. There's government and business passengers, which Belgium being the capital of the EU as well as Belgium although remote working practices may well be denting that traffic. There's also a lot of UK-Germany passengers who change at Brussels, although they are usually leisure passengers and business for that corridor will tend to prefer flying. Then there's reasonably well established mutual expat/migrant communities with members of families visiting each other - it's not massive but there's little Air routes between London and Brussels direct.
e300s tend to stay on the Paris route because only e320s can run to Amsterdam, although e300s will appear on Brussels terminators from time to time.
The option for intra-Schengen trips on Eurostar was temporarily suspended during COVID and permanently suspended shortly after, in order to enable faster journeys from Amsterdam to London (as the half of the train that was used for intra-Schengen passengers no longer needed a security sweep in Brussels).
Rotterdam is by far the best point to change for most other destinations in the Netherlands not being Amsterdam. Only North-Holland above the North-Sea-channel is more conveniently reached via Amsterdam, as Amsterdam Central has little connections to the north and east (though that would change when Eurostar eventually moves to Amsterdam Zuid)
