Either way, I see no use in introducing a comic version of a character just to replace them with someone completely new and never have them become who they are in the comics. Just bad writing. If they didnt want Laurel to become BC then they should have (they should have done it in the first place) and either got a different comic character or just gone with a complete original character.
The problem is that you can never foresee how things will ultimately turn out. They cast Rickards as Felicity, for only 2 planned appearances. And because she just blew everyone away, she became a series regular.
With Cassidy though, it's the exact opposite. She isn't really able to do much with the material she is given, and gets continuously outshined by the rest of the cast. The problem is also that she often gets things to do, that most of the time don't contribute anything to the plot. And when she does, she's basically relegated to a damsel in distress, or just there to cause relationship drama.
The writers have simply failed time and again to actually give us a character to root for. And I put the blame partially on Cassidy, for not performing well enough, to get us to care in the first place. Do you think Felicity would be as popular as she is now, if it weren't or Rickards' quirkiness and comedic timing? Good actors and can elevate weak characters through their performance. And bad actors can pretty much ruin great characters, no matter how well they are written.
You don't introduce Bruce Wayne and have some one else take up the mantle of Batman with him never becoming Batman.
Well, it's a good thing that Dinah Laurel Lance doesn't exist in the current main comic continuity, isn't it? We still have half a season to go. There is so much that could happen. Sara Lance is officially dead. She could be living off the grid for a while. Laurel could potentially die. And Sara could take her name.
She could either take her sister's full name, and integrate back into society as Dinah Laurel Lance. Or she just takes her first name, and takes a new last name, and goes by the name of Dinah Drake. The Black Canary in the New 52.
So, as you can see, if the name alone is so important to you, there are ways to get in line with the comics, that don't require to kill off the current Black Canary.