I cannot see where, in SC [Sacrosanctum concilium, the decree on the Reform of the Liturgy passed by Vatican II], there is a mandate for introducing Eucharistic Prayers alternative to the Canon Romanus. Nor can I see where it mandates alterations in the text of that Canon; least of all in the Words of Institution, traditionally treated with much awe.
The postconciliar rites expanded the 'table of Scripture' available; and SC had manifestly mandated this. You are at liberty to debate whether they did it in the best possible way; but you can't deny that they do have some sort of plausible pretext, in what the Council ordered, for what they did.
They expanded the number of Hymns in the Office; and SC had manifestly mandated this.
They reduced the Confiteors in the Mass from three to one, and Domine non sum dignus from six to one; and chopped out some Dominus vobiscums; unwisely, you are free to argue, but SC had explicitly mandated the elimination of some repetitions.
They provided at least (at my last count) thirteen, new, alternative Eucharistic Prayers: and where, pray, is that mandated or even hinted at? And yet those other matters, where the postconciliar changes rested upon and had apparently required a conciliar mandate, were peripheral in comparison with the importance of the Canon.
I do not see what Auctoritas there can be for the innovation of providing alternative Eucharistic Prayers. I think Bad Marini claims somewhere in the book with his name that the earlier custom of the Roman Church was to have alternative Eucharistic Prayers. But the Council Fathers nowhere mandated the erection of a committee with a free hand to resurrect anything it liked from earlier ages. They explicitly ordered that no changes be made which were not necessary; and that necessary changes should relate organically to what went before. In any case, it is unclear to me what Marini (or his ghost writers) means by his claim, unless it is some sort of speculative assumption about that period in the liturgical history of the Roman Church before written evidence becomes available. If he has in mind Eucharistic Prayers in use in the non-Roman Latin West, well, I have never felt that what I have seen of them gives much encouragement to go down such a path. Dix writes very well about how pre-Carolingian formulae compare very poorly with the Roman Rite. "Regret [for the demise of the Gallican rites] will perhaps be tempered for the student by the Gallican documents themselves, which plainly indicate that the end was not very far off when Charlemagne so abruptly hastened it. The barbarous boisterous Merovingian Latin in which they were composed would never have suited the clerks of the Carolingian renaissance ...". As far as these islands are concerned, we have, since the date of the earliest evidence, always been Roman Rite. The Stowe Missal, evidencing a form of the Roman Rite older than the changes made by S Gregory the Great in the 590s, suggests that even the 'Celtic' areas used forms of the Roman Eucharistic Prayer.
Of course, the new EPs are perfectly valid (ultratraddies who claim otherwise are simply manifesting their own illiteracy in the field of traditional Sacramental Theology). So what?
The Canon, the whole Canon, and no substitute for the Canon.