As Phil Klemmer says in the interview: "It would probably kill Tala Ashe to do all of those makeup changes constantly and having the poor camera crew do little motion-control tricks and split-screens and all the rest of it — it's tremendously painstaking, so yeah." It would never have been practical to keep them both. Okay, other shows in the past have managed to pull off the twinning trick (The Patty Duke Show, e.g.), but here it would be on top of the demands of all the other characters and action and special effects, and it would just be an added complication. And as Klemmer told TV Line, keeping Zari 1.0 or merging her with 2.0 would be like saying there was something wrong with Z2 and she needed the original to balance her or fix her. And Z2 ended up remembering a lot of Z1's skills and gaining her strengths, so essentially Z1 is already in her.