If people could opt-out there would be cold hard factual evidence a lot of people prefer not to use it, meaning the dev time spent on it might be called into question.
Instead they discontinue the old ui and can claim everyone uses the new one and as barely anyone comes to the forums they can also claim only a fraction of a percent of the playerbase has issues with the new ui thus it is considered a success and dev time well spent.
This justifies their salaries and shows the investors they are fulfilling their jobs.
If you could opt-out of the new ui this would be jeopardized thus they can and must not allow that for their own sake.
So if you are wondering why, this is their reason.
