Naturally, the sola scriptura advocate will deny all this. But the problem is that even the purportedly more modest, non-simplistic version of sola scriptura has no non-question-begging reason for denying it. The position is entirely ad hoc, having no motivation at all other than as a way of trying to maintain rejection of the various Catholic doctrines the sola scriptura advocate doesn’t like, without falling into the self-refutation problem facing the more simplistic version of sola scriptura. It is nothing more than an expression of one’s rejection of those Catholic doctrines, and in no way provides a rational justification for rejecting them.
I commented on this once before, but now I'd like to expand on my analysis:
i) Suppose there are ad hoc elements in the traditional formulation of sola scriptura. That, of itself, doesn't imply that sola scriptura is wrong. It may only mean we need to refine sola scriptura.
ii) You aren't required to have an alternative on hand to know that the status quo is wrong. Take Newtonian physics. That was a very powerful theory. But increasingly, there were discrepancies between Newtonian predictions and empirical evidence. At first that might be chalked up to inaccuracies in measurement. To the imprecision of telescopes, &c. But as technology advanced, and discrepancies multiplied, that fell outside the margin of error. Moreover, because Newtonian physics was such a tight-knit theory, it couldn't be tweaked with little fixes.
A 19C scientist could see that something was wrong with Newtonian physics, but not have a replacement theory waiting in the wings. For instance, Einstein's theory requires Riemannian geometry. But that wasn't available before Riemann.
Oftentimes, scientists don't begin with an alternative theory. Rather, what motivates them to explore alternatives is when the dominant paradigm becomes unsatisfactory.
Likewise, even if the Protestant Reformers didn't have an off-the-shelf alternative to Roman Catholicism, they'd still be able to see that Roman Catholicism was fundamentally flawed.
iii) Even if the Protestant Reformers had to improvise, the church of Rome has been improvising from the get-go. The church of Rome has been resorting to quick fixes and big fixes for centuries. Newman's theory of development retrofitted Catholicism. Vatican II retrofitted Catholicism. It's all about "saving the phenomena."
iv) That said, the Protestant Reformers didn't have to start from scratch. They had the whole Bible at their disposal. Likewise, there were pioneering theologians like Augustine, Anselm, and Aquinas whom the Protestant Reformers could cannibalize for spare parts.
v) Protestant theology didn't fall out of the sky with Luther. There were precursors like Wycliffe and Hus.
Luther's 95 theses weren't especially revolutionary. In his time, these were open questions in theology. It's Trent that locked Catholicism into certain positions.
vi) Papal supremacy has always been controversial. It was still controversial in the 19C, when Ignaz von Döllinger, greatest Catholic church historian of the day, opposed it. More significant was the number of Roman Catholic bishops who opposed the formal declaration of papal infallibility.
Papal infallibility was always controversial. Indeed, there are persistent allegations of heretical popes, viz. Liberius, Vigilius. This goes back to the patristic era.
Of course, papal apologists labor to extricate these popes from the charge of heresy, but that's irrelevant. My point is not whether they were, in fact, heretical, but the fact that misgivings about papal claims antedate the Reformation by centuries.
Same with respect to papal primacy. Consider the Quartodeciman controversy, or the dispute between Cyprian and Pope Stephen. Protestant Reformers didn't invent the wheel when they denied papal claims.
vii) Moreover, this isn't confined to outsiders or opponents of Rome. There's medieval conciliarism, according to which a general council outranks a pope. That was supported by Catholic theologians like Jean Quidort, Jean Gerson, and William of Ockham, as well as Cardinal Pierre d'Ailly.
viii) Furthermore, it wasn't just hypothetical. The Great Schism made that a practical necessity. The Roman church could hardly tolerate two or more competing, independent lines of apostolic succession, with each "pope" creating bishops. That had to be put to a stop.
The problem wasn't, in the first instance, that none of the claimants was the true pope. The problem, rather, was that even if one of them was the true pope, if it was impossible to tell which was which, then not knowing which one was the true pope was worse than having no pope at all. There was no way of knowing who to follow. What if you disobeyed the true pope by unwittingly yielding to an anti-pope?
To end the chaos, it was necessary to wipe the slate clean and start from scratch by deposing the claimants, if need be, then holding a new election with an undisputed winner.
That expedient succeeded, but at a cost. It was a stopgap measure. How can the pope be head of the church if his fellow bishops can depose him, even if he's the legitimate successor to Peter?
Typically, to be authoritative, a general council must be convened by the pope and confirmed by the pope. But, of course, that remedy was unavailable during the Great Schism, so the Council of Constance had to do it backwards. It was up to the council to ratify the pope, not vice versa.
ix) And the theoretical dilemma continued into the Counter-Reformation, with Catholic theologians like Suarez and Cardinal Bellarmine debating what recourse there'd be in the event of a heretical pope. They viewed a general council or the college of cardinals as the fallback.
This is emanating from doctrinaire supporters of the papacy. Papal loyalists. At the time, the raison d'être of the Jesuit order was to defend the papacy. But even so, they were forced to revisit the intractable conundra generated by the papacy.