While I don't mind that we're not getting to see more of the abusive relationship just to show what that's like because I think you can just as well start with her breaking up with him and then make the whole movie about her. It's not this great a loss, specifically, when it comes to story. Of course we'll see how that turns out in the finished film.
But I do think it's a pity that Leto is completely out. With a better script and direction, we still could've had a great Joker. I mean sure, not all of Leto's work was great but the sheer intensity he brought to it was great. So were the scenes of him and Robbie at Ace chemicals and the one where they fight on the road. It could've been something really good.
I mean, nothing prevents them from doing flashbacks to give the general moviegoing public more of a sense of what their relationship is like. I'm not saying we need to see a 'Carlo beats Connie'-style sequence but in essence Harley's relationship to the Joker is an abusive relationship where he beats her and then lies to her and she comes back to him. Even if it's a very uncomfortable topic, it could also be a really powerful story of how she leaves that monster behind for good. I also don't think it's a thing that should just be brushed aside as something that's easy to get out of. To just have her cut ties like that seems to do somewhat of a disservice to the importance that this kind of topic deserves imo but again, it depends on how it's done and how they're going to approach this.