Why PS just doesn't matter
I always find it a worthwhile exercise, when considering any theological or philosophical idea to ask the question of what actual difference it makes to our conscious experience of life. In other words, what is its "cash value" in the real world in terms of actually making any sort of difference at all? When I examine the concepts of Christianity with and without Penal Substitution, I can't seem to find any actual differences.
To say that "God is loving and so forgives out sins when we repent" seems functionally equivalent (in terms of how it ties into other doctrines and experiences) to saying "God doesn't forgive sin and demands punishment, but out of love sent Christ to take our sins onto himself, and therefore 'forgives' our sins when we repent". Similarly to say "God decided at the beginning of time that he would perform a final eternal judgment based on faith in Christ" seems functionally equivalent to "God decided at the beginning of time he would perform a final eternal judgment based on righteousness, but no humans could possibly achieve the required standard, but anyone who has faith gets righteousness imputed to them from Christ and thus passes the final judgment because of their faith in Christ."
In other words, the exact consequences to us and experiences of a penal substitutionary system seem to be able to be replicated without all the penal substitutionary doctrines being there. Thus if you isolated the penal substitutionary system as a 'black box' and just consider the inputs and outputs to the system (ie its functional effects), then you can't actually tell just by looking at how the black box is working whether it's penal substitution there or not. Someone could switch out the entire black box and replace it with another black box filled with non-PS theology that performs exactly the same function within the wider theological system.
What does this mean? Well, on the one hand, it doesn't affect the truth value of penal substitution (ie whether it is true or not). Whether PS is true or false is totally independent of any functional analysis about how it interacts with observable experience. But on the other hand, if something is functionally equivalent to its own non-existence then it is of zero importance and relevance. This is why in Science a really important thing about a theory is that it's testable. If it is functionally equivalent to other theories then it's totally impossible to tell which theory is actually the true one.
PS in my estimation seems to come pretty close to being functionally equivalent to a theology that contains no PS. The implication of this is that it is not an important doctrine. It might be true, but it isn't important that it's true. It's truth does not have effects on our lives that are any different to the effects its falsity would have on our lives. In fact in my experience it is fairly common among the average evangelical to hold all the standard doctrines (salvation by faith, good works flow from true faith, eternal life for believers, forgiveness upon repentance) which are themselves functionally equivalent to PS, without bothering to add PS on the grounds that "it doesn't make any sense" (as one boy on TV describing his faith put it). The tendency is to embrace the evangelical paradigm in general and ignore PS because this is perfectly doable, and for people who don't see PS as making logical sense it makes sense to swap out PS and maintain functional equivalence.
So, in summary, I do not think it can be validly claimed that PS is an important or central doctrine within the Christian faith, when it can be so easily in theory and practice swapped-out for other ideas. Whether PS is actually true or not or is taught in the bible or not is something we have to decide for ourselves. But whichever is the case it makes no direct difference to our wider theological paradigm and to our everyday experiences, because PS is very close to functionally equivalent to its own non-existence, and as a result of this is simply unimportant. The difference between "a God who is loving and forgives sins out of love" and "a God who demands justice be repaid but removes this need from himself by Jesus and thus forgives sins out of love" lies only in the semantics, logic and character of God depicted within this statements and not at all in the resultant functionality of these two doctrines or how they relate to our everyday experience of life.