The idea of streamlining is meant to include the number of books and publications, as well as the rules, so the possibility that there could be codices published, which subsequently have changed made by a Chapter Approved publication, would undermine the concept of simplifying the game. This was one of the reasons why GW did away with Chapter Approved and rules in White Dwarf in the past. The idea was to stop players having to reference so many publications. They're just reinventing the wheel here.
Had GW published all the unit data sheets and points values for all the armies free online and provided online amendments, and then left all the narrative and other material for published codices for players who wanted to pay for them, this approach would be acceptable, but if, and I do stress if, it's going to be necessary to buy codices and Chapter Approved, on top of the cost of the indices for those players who invested in them, that really would be a retrograde step.