Let's remember that Starc is considered the specialist for blowing away the tail.
When his side bats, he and the tail contribute a lot more runs than the opposition. Let's take this match as an immediate example of how the final score put pressure on SA.
Slagging off the importance of a tail destroyer is easy, until they score runs that impress the advantage. In the first innings the #8 Cummins got out leaving the score 251/7. The next two wickets added 90 runs, and the last added 10 runs.
Beyond stats, Mitch Marsh who batted with the tail didn't expect Starc to turn up and blow the AUS tail away, so he didn't farm the strike and just batted as if they were inexperienced batsmen. Yes, Starc isn't a tail ender himself, but could he and their bowlers face reverse swinging balls that might crush your toes? Break some other part of you, when very few actual batsmen have the technique to play against a moving ball?
In contrast SA's last 5 wickets posted a total of 12 runs. Yes, the SA batsmen failed, but Starc often destroys hopes of rearguards with swing and pace.
Australia raced ahead because SA don't have Starc bowling at the last 3 batsmen. Acting like blowing the tail doesn't matter suggests that maybe you're less interested in watching cricket than theorising from numbers.
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gone ! 
:
.png "Kumar Sangakkara :sanga")
has scored two 50's already