Scar made the effort to knock off both Mufasa and Simba. However, it seems that Zira only was mostly concerned with killing Simba. Had Kiara not been there when Kovu was planning to attack Simba, the most Kovu could have gotten away with before getting discovered was killing Simba. And he might have paid with his own life for doing that.
Had Simba died, I highly doubt the Pridelanders would have just given up and made Kovu king. They likely would have made Nala (because her children weren't of age and there was no known male lion old enough to rule), Kiara (as the firstborn), or Kion (because he is male) the new leader. Then Zira would have to do it all over again, and, since the first attack did succeed in getting someone killed (unlike Scar's first attempt, which failed), the new leader would be heavily guarded and very hard to assassinate.