But they didn't need to use Daneel or Seldon! It's possible to write an excellent story in the Foundation universe without using Daneel and/or Seldon. Donald Kingsbury did it. Benford, Bear and Brin are great authors, among my favorites, and could've added greatly to the universe Asimov created, rather than edit it as they did.
I think they were constrained by the same problem Asimov faced in 1986 -- where do you go after Foundation and Earth
? He had made the Foundation too powerful by 498 FE (which is one of the problems the galaxy faces in Foundation's Edge
; the Foundation can militarily achieve a Second Galactic Empire without any great difficulty, five centuries in advance of when the Seldon Plan says it can). Asimov himself didn't know, that's why he went back to explore Hari Seldon's life.
I also think that there were doing things to set up future Foundation books that, for whatever reason, haven't happened. Asimov (chronologically) leaves things at a point where the reader assumes that Galaxia will triumph. Brin makes clear that it's the Foundation that will ultimately triumph -- but he doesn't explain how
. I think the Second Foundation Trilogy was designed to get readers to that point, to write the series out of the dead-end that Asimov had written it into. If so, then subsequent books, which would necessarily have to take place post-Earth
, would likely explore the conflict between the Foundation (representing free will and individualism), the Second Foundation (representing predestination), and Gaia/Galaxia (representing collectivism) and how the Foundation ultimately triumphs over both. The problem is, we haven't seen those subsequent books.