I agree that her arc does make Batman more interesting. If all he ever does is beat people up and throw them into a mental health facility he knows to be abusive and under-regulated, he's just making things in Gotham worse. Someone with his intelligence and resources should be able to figure out how to do better, and Harley's rehabilitation does open up another future where Gotham's most unhinged and volatile metahumans and vigilantes have become productive members of a colorful and distinctive society.

Obviously that's not why she was rehabilitated. She's just significantly more popular as an antihero than she was as the Joker's sidekick. Arguing that she should return to how she was is like arguing that Deadpool should go back to terrorizing the New Mutants. People love these characters, and their love for these characters is why we keep getting to have all this high-budget content about them. Generally, as participants in this subculture, we should probably be happy when the characters evolve in a way that allows more people to connect with them. What we gain far eclipses whatever we lose in having Joker's girlfriend be a character called "Punchline" instead of "Harley Quinn."