Except Terri's rights and by extension Becky's, weren't terminated. Technically Hopper is El's legal father, not adoptive, as he is named as the father on her birth certificate while Terri is now named her birth mother, which reinstates her rights. Because Terri's is permanently disabled, Hopper's "rights" supersedes Becky's, but once Hopper "died," custody should reverted back to El's biological maternal family as Becky is now the closest kin to her. While I think Becky would agree to Hopper having El as his daughter because she had spent a year with him, he is better suited to protecting her from the people that initial took her away and El wouldn't want to be parted from Mike, no matter how nice Joyce is, Becky would now want El in her house in the room that Terri lovingly decorated and kept for her, especially now that Joyce is moving El away from Mike, her boyfriend, and now Max, the only girl that she made friends with, not to mention Dustin and Lucas who helped hide her when she escaped. And I would like to think that for a man that had already lost a daughter, and since Becky is stable person with a house, I would think he would want to give Becky and Terri the girl that Terri spent years fighting for.