Thanks.  I just noticed the discrepancy.  
For the record, the false information in the OP of this thread came from the blog, which is Dr. C's.

If you say "in perpetuity" means no Pope could make changes to the missal (that's what Pius V promulgated in perpetuity), then Pius V himself violated the law he enacted, since he changed the missal a few years after promulgating Quo Primum by adding the Feast of the Holy Rosary to the missal.  And Pius XII made huge revisions to the Missal when he revised Holy Week.  

So, either "in perpetuity" doesn't mean what you think it means, or Pius V and Pius XII (as well as other Popes) have done what the law forbids and have incurred the wrath of Almighty God and the Blessed Apostles Peter and Paul.  

So, explain why you believe in perpetuity, in the context of a law established by a Pope, means what you think it means.  If your answer is that you are simply applying the word based on your understanding of what it mean, that's not good enough.  You need to show that when the Church uses the phrase in the context of a liturgical law it means no future Pope can change or abrogate it.  

"In perpetuity", means "forever." In perpetuity is how long the law itself remains in effect. The law states that the Roman Missal ("this missal") "is hereafter to be followed absolutely." Anything that lasts forever is unchangeable, the law is unchangeable.  

So to use any other missal since then till forever is against the law of Quo Primum.

Along with all things Catholic, popes are bound to protect and defend whatever unchangeable laws there are that were put in place and remain in force forever for the purpose of protecting the Liturgy forever - that's why the law is there.  That's the pope's job, that's what he does, there is no one else who does that. Who else is going to defend and protect it?

What you are saying is it is ok for popes to break this law because they can, they can break it in order to do whatever they want to the Liturgy, even if they want to destroy or replace it - for no reason other than future popes are not bound by their predecessors.  

If a Blessing was used by the true religion during the time of the Old Testament, why would you concluded based on that fact alone that it is cursed and blasphemous to use today?…

“§ 712 It [the Holy Catholic Church] firmly believes, professes, and teaches that the matter pertaining to the law of the Old Testament, of the Mosaic law, which are divided into ceremonies, sacred rites, sacrifices, and sacraments, because they were established to signify something in the future, although they were suited to Divine worship at that time, after our Lord’s coming had been signified by them, ceased, and the Sacraments of the New Testament began; and that whoever, even after the passion, placed hope in these matters of law and submitted himself to them as necessary for salvation, as if faith in Christ could not save without them, sinned mortally. Yet it does not deny that after the passion of Christ up to the promulgation of the Gospel they could have been observed until they were believed to be in no way necessary for salvation; but after the promulgation of the Gospel it asserts they cannot be observed without the loss of eternal salvation. All, therefore, who after that time observe circuмcision and the Sabbath and the other requirements of the law, it declares alien to the Christian faith and not in the least fit to participate in eternal salvation, unless someday they recover from these errors….
 
“§714 The Most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews, and heretics, and schismatics, can ever be partakers of eternal life, but that they are to go into the eternal fire ‘which was prepared for the devil, and his angels,’ (Matthew 25:41) unless before death they are joined with Her; and that so important is the unity of this Ecclesiastical Body, that only those remaining within this unity can profit from the sacraments of the Church unto salvation, and that they alone can receive an eternal recompense for their fasts, alms deeds, and other works of Christian piety and duties of a Christian soldier. No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved unless they abide within the bosom and unity of the Catholic Church.”

Substantially changing the Missal into a new missal is breaking the law.

QP makes no mention of "substantial changes". You're just inventing your own rules now.