I do think Bishop Williamson might be thinking back a bit here.  It's certainly POSSIBLE that there are some old-timer priests out there, but they're getting very old.  I still do have a list of priests from the Cleveland Diocese (from their Directory of Clergy), most of whom now are retired and simply "in residence" at various parishes, etc.  And fewer and fewer are still active.  I keep this list in case someone has an urgent need for the Sacraments and no Traditional priest is around or available.  I went to a Jesuit High School in Cleveland, and one time my brother was kindof sneaking around the "Jesuit Residence" side of the complex, and he told me that he saw a little altar there set up for the Tridentine Mass.  Of course, that was nearly 40 years ago now.  When I was at The Catholic University of America, in the mid-1990s, I found an elderly Dominican priest to whom I went to confession.  I would bring him an enlarged printout (since his eyes were not great) of the Tridentine form for Confession and Absolution, and he seemed quite happy to use it.

Good to know that you have some experience in what +W said, regarding older priests still saying the TLM, and that you keep a list in case they are needed. I wish +W had elaborated further when he mentioned that there are underground networks of priests who celebrate the TLM, which led me to believe that there are other priests, other than the older ones, who are saying the TLM secretly. It wouldn't surprise me if it were true. Maybe that's only in the U.K.

+W also said, and many would probably agree, that Francis can't do away with the Old Mass, no matter how hard he tries - it's too late for that now.

Quo Primum is disciplinary, and for proof of that you simply have to look at the subsequent texts that follow it in a priestly missal, where the Popes tried but failed to get printers to revert back to what Trent codified with extensive use of Vetus Itala instead of the Vulgate but gave up, and left it be. Vetus Itala did survive with the Pater noster as the most famous example. The words of Quo Primum are conventional for the Papal chancellery, and they were ignored by printers. It still doesn't make the Novus Ordo something Catholics should follow. Trent itself accepted local liturgies of at least 200 years standing and while, say, France in its Neo-Gallican era of 17th-18th century ignored that, these other old liturgies and new liturgies were utterly Catholic which cannot be said for the Novus Ordo, which took an already short liturgy (Greek ones was take hours) and shortened still more and introduced ambiguous propers and offertories as the traditional one was supposedly 'medieval.' The less said of the lectionary the better.

As you know, I disagree that it's PURELY disciplinary.  There's a definite doctrinal aspect to it ... similar to how St. Thomas says that canonizations have a doctrinal aspect to them and therefore can be said to be "infallible", as infallibility in the strict sense can only refer to doctrinal propositions.

But my point earlier was that it's disputed even among Traditional Catholics, so that it would be an exercise in futility to attempt to prove it to OP's husband.  I didn't mean to start a full-on debate about it, just wanted to touch on the fact that there's no "smoking gun" that would convince OP's husband.  I do know some Traditional Catholics who hold that it's completely unchangeable and irreformable.  I hold an in-between position, that it's permitted to offer this Mass in perpetuity and that permission can't be revoked by a subsequent pope (any more than a subsequent pope could undo a canonization or a dogmatic definition).  This does not mean it can't be slightly modified per accidens.  Nor does it preclude a pope from introducing or promulgating an alternative Rite (although one could ague whether they could concoct a new Rite out of thing air vs. relying on a development from an Apostolic Rite).  In other words, I think this is controverted.  I just don't believe that a Pope could solemnly declare that the Rite is permitted in perpetuity but by that really mean "until I die and the next pope comes along."