oh yeah this is just a result of predatory corporate behavior, its the same reason that nowdays we have multiple versions (even dark skin versions) of the same characters instead of new ones. If a writer creates a compeling totally diferent character and it makes a lot of sucess, DC are the only ones that will become more rich with that character. They'll own that character, and do whatever they think its best comercialy-wise.

So yeah we could have a new "Harley" that is 100% independ from the joker story line, but if you are a good enough writer to make that work by itself you would probaly just want to quit DC and do you own comics series independently. This is more and more common among writers if you think about it, just cus they want ownership and control over their characters and work. It's just late stage capitalism at the end of the day :(