I have done Eurostar/Thalys from London to Cologne and I do not find the change at Brussels too troublesome. Quite honestly, going to/from Brussels is not much different than taking the train from/to DUS or CGN airport from Cologne railway station to connect with a flight (only that I sit in a comfortable train to Brussels rather than a local train serving the airport). Assuming that one has to get to the airport by public transport anyway, to change once at BRU is not a big deal if the schedules are sorted out to allow decent connections.