I know you're going to think I'm nuts on this, but at least Scar was a usurper and his reign was cut short and replaced with Simba's.
However, what if the cub Simba we saw before the stampede actually kept on that path, and perhaps even grew worse, as he got older? What if he ended up like Cusco from Emperor's New Groove, a self-centered, selfish, pampered ruler?
Unlike with Scar, who wasn't the true king, Simba would have been. As Scar himself admits he was too weak in the "brute strength" department, even if he outlived Mufasa and saw Simba rise as the new king, unless he had a covert assassination scheme, Simba would have remained the king.