From looking at google maps some pretty serious electrification hardware has been built on the trackbed of the southbound slow line, just north of Brant Green, so reinstating that line would be pricey.
My crayons had said to keep the Redditch lines on the east side of the through lines then cross the Birmingham bound line over a flyover in the open country to the north. Even without the flyover maybe moving the junction north away from the platforms would have increased the speed of trains across the junction.