"Pope is the supreme judge in the Church on earth, his rulings are final. You can try to dismiss it as a “mere surmise” but he made a very clear statement in a very clear letter that was very formal."
Pope St. Pius V's Quo Primum also contains "a very clear statement in a very clear letter that was very formal".
Any reasonable person reading that statement (below) would conclude that that pope authorized any priest to say Mass according to the 1570 Missal forever.
"...in virtue of Our Apostolic authority, We grant and concede in perpetuity that, for the chanting or reading of the Mass in any church whatsoever, this Missal is hereafter to be followed absolutely, without any scruple of conscience or fear of incurring any penalty, judgment, or censure, and may freely and lawfully be used."
Yup. It's clear to me that Pius V made his statement, and Paul VI, a successor to Peter with equal authority, made his own statement, too.
I don't happen to like the NO and it has been more troublesome than any other fruit of the Council--but the authority of the Pope cannot be questioned.