Meg, you are correct that one Pope's Motu Proprio can be abrogated, in whole or in part, by a future Pope. The Motu Proprio is similar to a regulatory rule. Traditionally it was understood to apply to the regulation of matters in the Papal States. Post Vatican-II it has been used as the vehicle of choice for various rules which the Vatican-II Popes understood would be modified by their successors. So the expectation of a future modification was built-in by using the form of the motu proprio. This was to get the laity into the dialectical back-and-forth that we see with the post Vatican-II era. Two steps forward and one step back. And on and on.

However, the Pope is NEVER above the canon law. Canon Law contains three elements: natural law, divine law, and human law. The Pope is always bound by natural law and divine law. He cannot legitimately change canons that reflect natural or divine law. He can, however, change, using proper legislative procedures, any human law in Canon Law. But until he officially changes the law, he is bound by the current law.

And some statements by previous Popes cannot be changed legitimately by their successors. These statements touch on infallibility when the Pope in question explicitly invokes his Apostolic Authority and binds the Universal Church in perpetuity on matters of Faith and Morals.

Finally, a Pope can never promulgate an unjust or unreasonable dictate that has the binding force of law. Paraphrasing St. Thomas Aquinas, "an unjust law is no law at all." See Article 4 on this page of the Summa Theologiae:

https://www.newadvent.org/summa/2096.htm