So basically the only differentiation is time and a story that makes it a mantle. So basically...any and all comicbook superhero personas can become mantles to include Nightwing.
No, the differentiation is that one identity waa created with the idea of
"Boy has life changed by crime, becomes nightmare of criminals"
And the other is
"Young crime fighter feels crushed by shadow of mentor, becomes his own hero to escape it."
Nightwing being a title passed down from former Robin to former Robin misses the entire point of the name.
Do I think it can't/shouldn't be passed on? Hell no. But the idea of it being a step on the way to Batman is awful.
If it's ever picked up again it should be in the same spirit. Some young hero looking to find himself sees the name, goes to the last person to use it to respectfully ask permission, then becomes a hero unaffiliated with that one.
Alternatively some punk kid picks it up without asking anyone then eventually proves she's every bit as worthy as anyone else to have it.