Then I guess what the script suggests happen is that B4 be developed, not (a new) Data?
Yes. B-4's whole role in the story was to be a flawed reflection of Data in the same was that Shinzon was a flawed reflection of Picard. Both Shinzon and B-4 were trapped because they were unable to grow and improve the way Picard and Data had always striven to do -- Shinzon because he blamed all his problems on his circumstances rather than taking responsibility for himself, and B-4 because he was simply too limited a prototype. The point of the engram-download scene was that it failed -- that B-4 couldn't become Data even with all Data's knowledge dumped into his head. Sure, the studio surely insisted on leaving a back door open just a crack, a way to bring Data back if the film had done really well and they'd wanted to do another with Spiner in it, but that was what the studio wanted. I think the intent of the screenwriters was the opposite of that. B-4 wasn't meant to be a vehicle for Data's resurrection, not primarily; he was meant to be Data's legacy. The knowledge Data left him would finally permit him to begin growing past his limits as he never could before, and that would be Data's final gift, allowing him to live on in the sense that we live on through our children or the people we help and inspire.