Okay, so just to play devil's advocate: it's entirely possible that some people do want that. Let me explain. When I first heard about Arrow, I barely knew anything about any of the characters. I knew Oliver Queen was the Green Arrow and who "Black Canary" was, but I didn't know her real name. So when I went through S1, I knew Black Canary was in it but I didn't know her secret identity (except I knew it wasn't Felicity, because the first clip I saw had both Felicity and–unknown to me–Sara in it, but I didn't know Sara's real identity). So as I watched, I kept expecting Black Canary show up and I she was blonde and so I didn't think she was Laurel. Then I read a description about the episode with Oliver's girlfriend McKenna and I thought "oh, maybe this is Black Canary" but she showed up and wasn't blonde either. When S2 came around and revealed it was Sara I just assumed that Sara Lance was always Black Canary. Didn't realize until months later that in the comics, Black Canary was Laurel. So for a person who isn't into the comics but just enjoys the show, I can see them not caring about who the Black Canary is, so long as she's present.
Now of course, knowing now, I cared that it was Laurel, as it should've been, and I was upset that they killed her off. However, if I'd remained a casual fan, clueless as to her identity in the comics, I probably wouldn't have cared as much that they would replace her. Would I, as a casual fan, have been sad at Laurel's death? Yeah, for sure. But I would've also been okay with them just bringing in a new Black Canary, too. (BTW, back when I was still a casual, I also really enjoyed the idea of Olicity, but this was during S1 and S2, before they showed why it's a bad idea.) So I would say that, for the majority of casual fans, they probably will be okay with a new Black Canary (unless they're Olicity shippers, in which case they could feel threatened, even if unnecessarily, by yet another woman in Oliver Queen's life besides Precious Felicity).