I think they are smart enough to know that by moving everything to Origin, they may lose support from gamers due to availability of DLC and the like. I reckon they'll keep support and DLC available on Steam.
I would assume that any future Dragon Age DLCs would be an Origin exclusive as well, but there is no way to know for sure yet. Though aren't the DLCs purchased through the game anyway? I don't think Steam is even involved.
However, i think it is safe to say that there won't be any more DLCs for Dragon Age. Perhaps for DA2, but even that is unlikely. I think they've milked that market out, but who knows. There might be a new one.
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