Like it or not, Bp. Williamson's arguments do not pass the test of Catholic principles, as you can see below.
This is a crucial sentence in Bp. Williamson's talk:
"I would not say every single person must stay away from every single NO Mass."
So the question is why does a person need to stay away from any NO Mass in the first place - is it because of the substance or because of the accidents? The Traditional Catholic response is because of the substance (the NO is poison, it harms the faith, it doesn't express the Catholic Theology of the Mass, it is defective because of its omissions, even in its Latin form and said reverently etc.), and not merely because of the accidents not necessarily present in the rite itself (e.g. altar girls, Communion in the hand, lay extraordinary ministers of Communion, tables instead of altars, the priest turning his back on the Most Blessed Sacrament in the Tabernacle etc.). But if the NO Mass is defective and harmful in its substance (i.e. even when said reverently and in the most conservative Latin form), then it is not a Catholic rite of Mass, since the Catholic Church is unable to promulgate defective and harmful rites of the sacraments. And no Catholic is allowed to assist at non-Catholic rites (except in a civil capacity at funerals and weddings), and this is something that not even a pope can dispense from (popes have in extremely rare occasions permitted communicatio in sacris with heretics and schismatics to certain Catholics, but it was always Mass said in a Catholic rite, one of the Eastern rites).
Furthermore, to quote from the N.O.W. analysis:
"Did you notice what he left out? God. He did not think to ask whether the New Mass is pleasing to God. That is all that matters. Holy Mass isn't about us. It's about God. It's the worship of the Most Holy Trinity, not a spiritual pick-me-up."
Indeed, because of all the defects present in the substance of the NO Mass, I believe all of us would agree that it cannot be pleasing to God, even when said reverently. This means that to participate in it, even if it is said in the most reverent manner, is to partipate in an act of offending God, which we must never do, even though our intentions and the intentions of the priest might be of the best possible kind.
Therefore, since the NO Mass is not a Catholic rite and is not pleasing to God, if Bp. Williamson's sentence was based on Catholic principles, it should have said: "Every single person must stay away from every single NO Mass."
But it did not say that.
If we try to find the principles behind what it did say, what we would come up with is that the NO Mass is not always harmful and defective and that it can sometimes be pleasing to God, which actually means that it is not defective and harmful in substance, but only because of the accidents. And just as the late Michael Davies (traditionalist author and longtime president of the "Una Voce" Indult organization) we would then have to conclude that the NO Mass, at least in its most conservative form, is a Catholic rite. There goes our resistance to the Conciliar Revolution.
Therefore, Bp. Williamson's response is either against Catholic principles, or it is subjectivist (in the sense of nevermind the principles, just do what you think is right).
Both is troubling.