Eh, I very much doubt that Tommy was ever intended or was presented as being the Dark Archer. Malcolm Merlyn trained in Nanda Parbat. Tommy Merlyn got rolled over whenever he and Oliver came to blows. That poor guy had no chance at all.
Actually, the same problems with Tommy had with becoming the Dark Archer (totally untrained civilian completely outmatched by Oliver) apply to Laurel Lance when becoming the Black Canary. Oliver, and probably Sara, picked up their master level skills over five year of hardship and violence. Laurel, on the other hand, is a barely trained civilian who has never whipped out her "I'm a badass" hat without failing miserably. It's going to hard to buy it if, for example, Sara gives Laurel a martial arts crash course over a season, dies, and then Laurel picks up where she left off less four plus years of training and experience.