It is fair to note that out of all the positions PSA is the one that treats sin the most superficial (Satisfaction Atonement and Substitution Atonement treat sin too lightly, but not as bad as PSA).
This would only be true in some starnge "Alice through the Looking Glass" world, where everything is the opposite of what is true.
In fact PSA is the one doctrine that treats sin with the seriousness with which God treats it. It is also very, very simple, unless one is determined to complicate it.
'He was pierced for our transgressions; He was crushed for our iniquities.' He we have Penal Substitution at its simplest. We deserved to be pierced; we deserved to be crushed; that should have been the penalty for our sin; but that punishment was instead willingly taken by the Lord Jesus Christ. He was our substitute.
'By His wounds we are healed.'
There was none other good enough
To pay the price of sin.
He only, could unlock the gate
Of heaven and let us in.
Or as a medieval hymnwriter puts it:
Mine, mine was the transgression,
But Thine the deadly wound.' [I added that just to show that the doctrine of Penal Substitution was alive and well before the Reformation]
The Lord Jesus,
'...Poured out His soul unto death, and He was numbered with the transgressors, and He bore the sin of many.' And this was not just some random bloke plucked off the street; this was God the Son - the sinless One numbered with the transgressors, bearing their sin. The seriousness with which God treats sin could not be more clearly shown that in Penal Substitution.