The Eurostar service from London through the Channel Tunnel is hamstrung by the most extraordinary set of "security" measures imposed by a specific quango, the "Inter Government Commission", that make any airport manager thankful they are not in this particular business. The ever-increasing costs involved are a significant part of why Eurostar in the end chose not to open its station at Stratford (which would obviously be significant competition to LCY), and why it sits today as an empty shell on the railway.
There is no prospect of expanding the Eurostar service from London to any points other than the ones already served, as all this "security" stuff, plus the requirement to segregate the domestic passengers from the ones travelling to London will apply, so extending Eurostar beyond Brussels to Antwerp, Rotterdam, Amsterdam, Cologne, etc is not going to happen. So passengers will need to change trains at Brussels and go through all the hoopla of security there. Furthermore, as many of the fares (including the ones quoted above) are for specific book-ahead seats on specific trains, non-refundable/changeable rather than being valid on any train, and as the connections are by a different ralway organisation, the connecting passenger risks it all should their train from say Amsterdam to Brussels get delayed for any reason. The trains are not timed or arranged to connect there.
If you know how it is with a "connection" between two Ryanair flights at Stansted, that will give you a pretty good idea. So Eurostar is not going to make much headway in any market beyond Brussels.