Find your tribe in a Sea of Creativity
Nightwing is not a retired crashout. He's just a crashout on hold.
He, like any good eldest sibling with Eldest Daughter Syndrome, is letting all his younger siblings have a turn at crashing out. Anyone with Batman as their parent/parental figure deserves to. It's practically tradition at this point.
Sure, he didn't expect Bruce to keep adopting more and more, but okay fine. He likes having lots of brothers and sisters. He likes being a big brother. Once the flow of new siblings stops and they've all gotten it out of their systems, first chance he gets Nightwing is going to finally allow himself his second run of things. He's just waiting.
Bruce keeps adopting, partially because he can't help it, and partially because he'd rather deal with a dozen regular Bat Children going through the typical teenage rebellion phase than Dick Grayson experiencing a crashout literally years in the making. If he stops adopting he knows what will happen. At this point he's spent too long holding it off to hope the inevitable fallout can be contained.
Teenage Dick Grayson crashing out was bad enough. Adult Dick Grayson has been gearing up more or less for over a decade.
No one will be prepared, except maybe Jason, who has been gleefully waiting for the older brother he remembers to make a comeback since he first showed up as Red Hood, Tim, who is ankle deep into the beginnings of a villain arc at any given moment and spent most of his pre-robin years photographing everything that happened the first time, and Harley Quinn, a licensed psychologist specializing in crazy who has been watching the warning signs build up like an unstable jenga tower for years.