I know this following argument adresses that question in very broad way, but I'm curious for your take on what I'm about to write:
To me, "respect" for source material in adapting characters is tied heavily into making sure that whatever you're adapting matches the potential of its basis in some way; if you have an adaptation of Arthurian mythology, for instance, don't show a Merlin who doesn't some how impress the audience on par with the older versions of the character. Even if you decide to subvert some aspects of his portrayal, you almost always have to enhance other aspects to make the adaptation worthy; thus, a significantly smaller and less massive Bane can have a good adaptation in TDKR if his intelligence, skill, and charisma is presented to compensate.
Now, I've thought for quite some time that there's really two different portrayals of Black Canary prevalent in the comics: Black Canary as a supporting character to Green Arrow (Let's call her Lady Bird), and Black Canary as an independent superhero and member of the Birds of Prey and Justice League (Let's call her The Chairman).
Lady Bird was the more popular version of the character during the 70's to early 90's; and it's no coincidence that Dinah had several more Damsel in Distress and victimization moments in that time-frame. But once Birds of Prey took off, a new generation of readers got introduced to a more no-nonsense, sparring-partner-with-Lady-Shiva Chairman who didn't need any rescues and arguably came off better than Oliver as both a crime fighter and as a personable individual (makes sense; Ollie was dead for a good portion of the 90's and 00's!)
And I think that's why some people will always have trouble taking Cassidy's Laurel as a well handled adaptation of their ideal version of the character; Cassidy was always going to be Lady Bird, a supporting cast character who was supposed to have her love life and bond with Ollie define her. Cassidy did a great job, but the failure to get her chemistry with Amell's Ollie right made for a handicapped Lady Bird character who at times emphasized the weakness of that character direction...
...While at the same time, here comes Sara, meant to be a strictly ancillary character for the show, but kind of turning into a pitch perfect version of the Chairman for the show; she's portrayed as Ollie's equal, a take charge vigilante capable of coordinating attacks against the likes of Deathstroke, and defined much more by her charisma and sheer butt kicking ability than by her ties to Ollie.
And now, after being mishandled and executed by the show runners, Laurel seems to have officially faded away from prominence in the show, while Sara, a character who clearly grew beyond their expectations, is right now still following some of the notes of the Chairman Black Canary pretty successfully, and even getting a bit of a BoP partnership thing going with Hawkgirl.
Before I get into addressing your post, I'd like to post the following quote from Fellowship of the Ring editor John Gilbert, as it explains my philosophy on adaptation in a nutshell:
"I always find that literal adaptations don't work. You have to find what you think is essential to the book, and make your movie of that."
Now, on to addressing your post.
Given that I feel the way I do when it comes to adaptation, I have some very specific criteria for what does or does not constitute a "satisfactory" adaptation of something:
1) Does the thing being adapted appeal to me in and of itself
2) Is it consistent in its own internal narrative and narrative logic?
3) Is there a clear "endgame" in mind in terms of what's happening?
4) Can I still recognize the original source material in the interpretation I'm seeing?
Arrow's portrayal of Laurel Lance fits every single one of the above criteria, and I am therefore able to put aside all preconceived notions of what I want out of the character and just go along for the ride, accepting that this version of Laurel is intended to mirror Oliver more than any other character on the series and that we, as an audience, are meant to follow her as she passes through a "crucible" and emerges on the other side different from the person she'd been before. The introduction of a character like her sister Sara plays into that by introducing character traits that Laurel doesn't quite have yet that she'll need to develop in order to "become who she was born to be", just as the introduction of characters like Yao Fei, Shado, and Tommy Merlyn bring character traits into play that Oliver doesn't yet have but will need to develop in order to become the character that he's meant to be.
Arrow is and has always been a show that is very much like FOX's Gotham in that it's an origin series. However, whereas Gotham wears its "origin-ness" very much on its sleeve unapologetically, Arrow tends to be more subtle in its approach, building the characters towards an "endgame" that mirrors the comics while approaching things from a more real-world perspective where things aren't as clean-cut as they might be in said comics.
Audiences were never meant to look at the character of Sara and compare/contrast her to Laurel or go "they already have a comics-perfect Black Canary; why are they building Laurel up to be that character?" Sara's sole purpose, narratively, was to, as I noted, be a character who could help Laurel evolve into someone who was 100% recognizably the Black Canary from the comics. However, even with that "endgame" in mind, there was never any guarantee that we'd get to see her actually become the recognizable Black Canary from the comics, or that she'd reach that "endgame" quickly, just as there is no actual guarantee that Oliver will ever reach his "endgame" of becoming a Green Arrow who is recognizable from the comics.
We'll unfortunately never get to see Arrow's version of Laurel reach her "endgame" because of the decision to kill her off, but that in and of itself is reflective of the show's real-world approach to adapting the Green Arrow source material.
