Should human law always be changed when something better comes along?

It seems that human law should always be changed when something better comes along:

Objection 1:  Human laws, like other matters of art, have been arrived at by human reason.  But in other matters of art, what was previously embraced is changed if something better comes along. Therefore, the same thing should be done in the case of human laws.

Objection 2:  We can provide for the future by drawing on the past.  But if human laws had not been changed to better discoveries that superceded them, then many anomalies would have followed, because old codes of law contain many elements of ignorance.  Therefore, it seems that the laws should be changed as often as something better to institute comes along.

Objection 3:  Human laws are established with respect to particular human acts.  But the only way we come to complete cognition of particulars is through experience, which takes time, as Ethics 2 points out.  Therefore, it seems that through the course of time something better to institute can come along.

But contrary to this:  Decretals, dist. 12, says, “It is a ridiculous and wholly abominable disgrace for us to break off the traditions that we have received from our fathers.”

I respond:  As has been explained (a. 1), human law is justifiably changed to the extent that such a change in the law is a means of providing for the common welfare.  However, the very changing of the law, taken just by itself, does a certain sort of damage to the common welfare.  For custom (consuetudo) contributes to the observance of a great many laws, to the extent that whatever violates common custom—even if it is less important in itself—is seen as rather serious.  Hence, when a law is changed, the constraining force of the law is diminished to the extent that a given custom is nullified.
Therefore, human law should never be changed unless the damage done to the common welfare by the change is wholly compensated for in some other way.  This happens either (a) because some very great and obvious advantage comes from the new statute, or (b) because there is some very great necessity in virtue of the fact that the established law either involves some manifest iniquity or is such that its observance is very harmful.  Hence, the Legal Expert says, “In order to revoke a law that has been deemed just for a long time, there must be an obvious advantage in the new practices that are going to be instituted.”

Reply to objection 1:  Matters of art have their efficacy solely from reason, and so whenever a better reason comes along, what was previously embraced should be changed.  By contrast, as the Philosopher points out in Politics 2, laws acquire an especially great force from custom.  And so they are not to be changed so easily.

Reply to objection 2:  This argument shows that laws should be changed, but not for the sake of just any sort of improvement.  Instead, as has been explained, they should be changed for the sake of some great advantage or because of some great necessity.

Reply to objection 3:  The same reply holds for the third objection.