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I agree that they seem to be stretching keeping the cast to keep them a part of the show, when it SHOULD be about a long term saga. But it's so hard to let go of Jared Harris -- a great actor.
Jared Harris (Aka Hari Seldon) is the only one who keeps coming back in the books and so he's the only one that they don't have to make a special effort to keep around.
It wouldn't be too much of a stretch to keep him as a holographic advisor of the second foundation for example.