It is agreed that termination is expected, but the exact date of termination is the key issue. Finally, the termination date is finalized, and mentioned by OP (25 MAR 2018).

I agree that closing of DUS is a pity as it is the cheapest business and PEY fare from Western Europe, (Compared to CDG, FRA, AMS, ZRH)

For ex-DUS travellers, I would expect a substitute for Oneworld will be a codeshare with BA via LHR. Codeshare with LH group is another feasible option but details are to be confirmed later.

Rail service from DUS to BRU, FRA, or AMS will work as well, I think the train segments is generally less than 2 hours, even though it can be more than 1 train segment to BRU, FRA, or AMS.

Although it is true that Liege or East Belgium can have Zaventem as a substitute, the proportion will not be much.