Ok, great. If I understand you correctly, Caminus, the SSPX position is that the New Mass, owing to some reasons during its promulgation, is not therefore protected by the indefectibility of the Church.
Before we come to the liturgy itself, concerning the place of the rite itself in the Church, let me ask you, even if it was as you say, doesn't the subsequent approval of all Popes including Benedict XVI, most especially and manifestly in his motu proprio Summorum Pontificuм seem to settle the issue once and for all? I ask this only because it seems to me that, whatever my private judgment of the liturgical text may be, Catholic faith obliges me to hold this.
If we are to go into the text itself, well, I think there is no doubt that what Pope Benedict XVI calls "The Extraordinary Form" is a richer, more beautiful and more expressive manifestation of lex orandi, lex credendi. That said, I do not think "The Ordinary Form" fails to manifest with sufficient clarity the twin and inseparable character of the Mass as at once a sacrament and a sacrifice. I have read the famous "short, critical study on the New Mass" but I also heard that Cardinal Ottaviani himself was satisfied after one of Pope Paul VI's address regarding its doctrinal precision but called for an extensive catechesis to make sure the faithful understand the prayer and belief of the Church.