Why it's unthinkable to deny penal substitution

"It would, of course, have been unthinkable that God should go back upon His word and that man, 

having transgressed, should not die" 

Athanasius, On the Incarnation

And how will man be redeemed from death unless another takes his place and bears his punishment?  Forgiveness without substitution would mean that every word of God did not prove true after all.