In that case, due to the lack of a pope, the bishops would have exercised their functions with supplied jurisdiction and later been given ordinary jurisdiction by the new pope; however, that situation is not comparable to today’s situation, since in the space of those three years, there were still bishops remaining from the previous papacy that did have ordinary jurisdiction.
It isn’t the length of time itself that is problematic if we have had no pope since 1958, but the fact that if no bishops appointed by Pius XII are still living, all ordinary jurisdiction has disappeared, and consequently, the entire Ecclesia docens has disappeared. Cdl. Manning, a Father at Vatican I, wrote, “Even though a number of bishops should fall away, as in the Arian and Nestorian heresies, yet the Episcopate could never fall away."
Even if there were still one bishop with ordinary jurisdiction appointed by Pius XII left living, sedevacantism would still be a plausible theory, but alas, this is not the case. While not dogma proper, the belief that ordinary jurisdiction is an essential attribute of the Church and thus cannot cease to exist at any time is at least theologically certain (sententia ad fidem pertinens) due to its intrinsic connection with revealed truths concerning the nature of the Church.
Many sedevacantists turn to the principle of epikeia which essentially states that laws are for the benefit of the people and that a law that ceases to benefit the people becomes invalid.
That’s not quite what the principle of epikeia (equity) means; it refers to “the benign application of the law according to what is good and equitable, which decides that the lawgiver does not intend that, because of exceptional circumstances, some particular case be included under his general law.” —D. Prümmer, Manuale Theologiae Moralis (Barcelona: Herder 1949) 1:231.
St. Thomas Aquinas states that laws that are intrinsically unjust are not valid laws (Summa I-II, q. 96, art. 4), but this is distinct from the situation in which laws that are per se just (in principle) can be applied unjustly (in practice). As an SSPX supporter, I would apply this in the case of the 1988 consecrations; while canon 1013 of the Code of Canon Law, which prohibits episcopal consecrations without papal mandate, is in itself just, its application would become unjust in the extraordinary circumstances Abp. Lefebvre found himself in, and thus could be set aside.
The extraordinary circumstances did not make the law invalid, but only permitted temporarily setting it aside as, despite being per se a just law, the strict application of its letter would become unjust under the circumstances. This is what the Code of Canon Law refers to as a lacuna legis, where due to extraordinary circumstances, it becomes unclear how a law should be applied, so we ought to apply the law with “canonical equity” by following its spirit rather than the letter.