The problem are indeed the access charges, which, just like in France, make it prohibitively expensive for non-incumbents. The EU should do something about that.
But Brussels would be a good market. Especially if they time the train to have good connections with Eurostar and sell through tickets.
Changing in Köln and using Thalys has currently a few disadvantages. DB is no longer selling Thalys tickets for example, and in the last Thalys from Brussel is to early, meaning that you have to hang around Köln Hbf for over two hours late at night...