There are legitimate concerns that planning what one will do in the case of divorce is not compatible with the Catholic teaching that divorce is never an option.
You’re not distinguishing between CIVIL divorce (which the church allows in certain circuмstances) and divorce/re-marriage, which is never allowed.
God tells us that murder is a grave offense. Does that mean it doesn’t happen? Does that mean we shouldn’t take precautions to protect ourselves in dangerous situations?
God tells us that divorce/theft/re-marriage is a grave offense. Does that mean it doesn’t happen? Does that mean we shouldn’t take precautions to protect oneself from this evil?
If one commits the sin of murder the state steps in and sends the person to jail, as they should. Also, people have the right to self-defense and the right to kill one who is threatening their life.
If one commits the sin of divorce (assuming it's not Church sanctioned, which should be rare), the state does NOT step in and help achieve justice; they promote and encourage injustice and mockery of a sacrament and a contract. Does the spouse who is being divorced unjustly not have a right to self-defense and a protection of what is rightly theirs?
In the same way that guns/weapons deter criminals and promote safety, one could argue that a prenup could deter a temptation to destroy a marriage, which would be a great thing, and could ensure that spouses “work out their differences” and thus save them from hell.