the previous film grossed over $1 billion & the director, lady gaga, & phoenix were given 8-figure salaries for the sequel. It seems like this will the final film given what I've read about the ending. Besides being disliked by people who saw it but the premise & ending goes against what is established in the comic book universe.
The relationship between Harley Quinn & Joker is meant to be dominated by the Joker. The Joker never plays second fiddle nor does he ever really 'listen' to Harley. Harley is a totally enthralled follower, and obsessed with him. She does break up with him from time to time but always manages to relapse some how, and her break ups tend to intwine her with the Batman, which she has sexual tension with.
There are scenarios from comic book writers where the Joker does revert back to his old identity though. I think the mainstream audiences don't know that.
From what I gather the ending of Joker 2 is kind of a response to the controversy of Joker 1 in which critics were complaining that the film was glorifying lone-wolf violence & acts of terror. So it is a cop-out in a sense, and self- protective of the filmmakers.