But equal cannot bind equal, and "every one knows that the Church has the power to change and abrogate what she herself has established." (Sacramentum ORdinis, Pius XII). Therefore, Pius V and Pius XII did not exceed their authority when they changed the missal that Pius V promulgated in perpetuity.
You are on the total wrong track because the question of 'equals able or unable to bind each other' is not even the issue.
We are discussing the law of Quo Primum, put in place to protect the Missale Romanum which is by this law, the official Roman Liturgy, canonized as the only liturgy permitted for the Roman rite for all time.
No one is disputing popes may, after due diligence and explaining their reasons (which all the popes who did make any change to the missal did), make incidental changes to this missal. No one disputes this.
The issue is that the law was put in place to forever protect the central, most important part of the Roman Catholic faith, the Liturgy. Popes are the protectors of all things that in any way pertain the Catholic faith, including and most especially her Liturgy.
It is their job to preserve and to protect that which their predecessors have preserved and protected, and hand all of that down to future popes, who are also expected to the same - and on and on it goes till the end of time, this is how the Catholic faith has and will remain till the end of time. After Pius V, all future popes' obligation and duty is to protect and preserve everything, all laws, teachings, disciplines, decrees and everything else of all their predecessors, that's is what popes do.
Our Lord did not institute the papacy in order for popes to be inventive and to change, and change some more just because as the Church's supreme ruler on earth, no one can tell him he can't - that is an idea that is anti-Catholic.
So it is not about whether or not as pope, they are bound or not bound to the law, or have or have not the power to change the law, it is their job to uphold the law, as they alone have the authority to maintain the law, to protect it and preserve it for the future Church and popes - that is why they are there.