^ Where did you get the idea that Sara "wasn't supposed to be a Canary character"?.
From here:
http://www.tvguide.com/news/arrow-ravager-summer-glau-spoilers-kreisberg-1080585/. And let me be clear; I'm not arguing that Sara was meant to be a character completely separate from Laurel's transformation into Black Canary in any way; rather that the decision to have Sara act as an exact and direct precursor to every single visual (and at times, story) trait of the Black Canary was a decision made later than the decision to bring Sara back. And I feel that part for the issue with Laurel's value to the overall fandom was that having a better written and more exciting Canary character concurrent to a botched version of the Demon In The Bottle for Laurel set up a clash between fans who immediately wanted to see Badass Canary on a permanent basis versus those who wanted to see a highly modified version of the character eventually become Black Canary.
Now, I think the real issue with both Lance sisters is just the overall writing post-Season 2; there was no way that killing Sara off was needed to make Laurel transition into the suit, and now that we've had both sisters die, there's a feeling the writing team is being driven more by shock deaths then organic storytelling.
In spite of her popularity, Sara never actually had her own character arc(s) prior to her inclusion on Legends of Tomorrow; she existed SOLELY as a "prototype" character to service Laurel's evolution into the Black Canary, just as Yao Fei and Shado served as "prototypes" to service Oliver's evolution into the Green Arrow.
And I say this as a fan of both sisters.
I kind of disagree with this, and I'd argue it again exposes the weakness the creative team had with handling Laurel before her Black Canary-dom; Sara's character arc was a more subtle and organic transition from her mirroring Season One Oliver, but as an assassin who was arguably even more comfortable with killing, to a woman trying to follow Oliver's lead as a hero, while arguably seeming better adjusted, and literally so scared of returning to the assassins that she was willing to poison herself and die!
...And then the writers pulled a 180 on her in the last episodes of the Season and had her go back to the LoA smiling all the while. Kind of like how Laurel vengeance crazy in season 3 then became suddenly the rational hero when they wanted her to.
To me, the rough outline of Laurel's storyline, and my interest in Arrow, goes like this:
Season One: a generic but mostly well done CW love story with Tommy suffering from a badly handled transition to Oliver-Laurel-Tommy love triangle that nonetheless tied into the season's arc very well, so was overall pretty good.
Season Two: strong start for Laurel, but veering off into an almost entirely unconnected subplot that went far too long and looked horrific in comparison to the best season of the show.
Season Three: starting off pretty bad with the unnecessary and badly handled fridging of her sister, and then Boom! Suddenly Laurel's storyline is actually pretty great in comparison to the show becoming just as tired and derivative as her Season Two story was.
...and here's where I stopped watching the show regularly. I've caught a few episodes, but the writings become to disappointing for me to watch it without cable.
And now she's dead in season four, in what definetly sounds like a rather pathetic exit compared to stuff that other characters (e.g., Oliver himself) have survived.
It's already a CW staple. Plus, I highly doubt there's going to be a ratings dip anyway. The people who like her is still a minority IMO. This is one of the very few places actually that has grown to appreciate her. Other than this and maybe some Tumblr sites here and there, the majority still ranges from indifference to annoyance to hatred for her character. In other places, people are actually rejoicing that she's dead and will no longer be a threat to their favorite ship. Personally, I don't get that because it's clear that MG is Olicity4eva. Laurel stopped being a threat to that ship a long time ago. So yeah, my point is the majority got what they want and the majority will be enough to keep this show for 100 or more seasons.
