To judge by his behaviour (if not by his explicit statements) there seems little doubt that JRR Tolkien regarded the consequences of the Second Vatican Council with a combination of deep dismay and horror; indeed, I once wrote that Vatican II may have been the most deeply dismaying event of Tolkien's whole life.
His friend (and fellow Catholic) George Sayer; believed that Tolkien saw little or nothing wrong with the pre-Vatican II Roman Catholic Church.
This is worth contemplating because it shows that a Catholic as devout as Tolkien could be in deep opposition to Vatican II; but without making reference to "legalistic" aspects of the validity of Papal elections. I imagine Tolkien would have found that whole "sedevacantist" line of argument on both sides to be extremely distressing and fraught with dangers - especially when engaged-in by lay people.
This may have some implications for traditionalist Catholics who all agree that the RCC took a severe wrong turn, a down-turn and movement towards apostasy, with the Second Vatican Council.
My point is that the disastrous nature of Vatican II may be regarded as common ground among serious Roman Catholics. And indeed all serious Christians of any- or no-denomination, who wish for the recovery and renewal of spiritual health in the largest and most influential Christian church - this may be common ground quite apart from the legal arguments.
In other words; sedevacantism can reasonably be regarded as one of several (or many) possible hypotheses for explaining the disastrous effect of Vatican II; and, most importantly, how to set it right.
It is indeed possible that the sedevacantist legal arguments for why the papal seat is empty might be true; but the legalistic solutions may nonetheless be ineffective or counter-productive in solving the Roman Catholic Church's many and deep problems.
That is, indeed, my opinion - I mean that the sedevacantist solutions (i.e. their advocated approach to dealing with the RC problems) are ineffective, and would be counter-productive: they are wrong in their spirit.
Because I think it can be known in advance - from multiple experiences in multiple churches - that legal solutions will not have a good effect; that a re-set is impossible (and the attempt undesirable) because it will empower the wrong people and set faithful Catholics at each others throats...
That this negative potential can be known in advance from experience, and legalism eschewed, even despite that (probably) nobody has yet proposed any other clearly promising and practical way of genuinely revitalizing the Roman Catholic Church in the West.
For what little it is worth; I suspect that an answer might be found in the actual practice of Roman Catholicism at its Christian best; rather than in abstract theories about the matter.
Something to do with the lives of ordinary Catholics (including 'ordinary' Saints); rather than the models and hypotheses of canon lawyers, theologians, philosophers, church bureaucrats or the like.
Maybe the most luminous, rich, and inspiring aspects of the pre-Reformation Catholic 'world' - something rather like GK Chesterton's imaginative pictures of "Merrie England" - could be found to contain clues toward the changes that are needed and would work and could grow; and also (and vital) what aspects ought to be de-emphasized...
Selected cuttings from that ancient tree of faithful living might be planted to yield new and different fruits, but recognizably derived from the same root stock.