Third, if you could realistically use only the Bible alone to judge every question, then you could just strictly quote Bible verses to another open minded Christian with an opposite point of view whenever you want to both decide the Bible's position on any topic. You would not add your own sentences to what you quoted from the Bible to explain what it means, because when you do, you are using some source (your own) in addition to what is found in Scripture. So if someone asked whether to baptize infants, you could just quote to them from the Bible and they know what to do. But in practice that doesn't work. A Lutheran can't in practice just go to a sincere Protestant who denies infant baptism and without any explanations only quote the verse where Jesus says, "Let the children come to me," and the other Protestant will get the Bible's position on the topic, because the verse is not on-point enough on the specific question of baptizing infants. For instance, maybe the children were not "infants."
Fourth, in practice Luther, Calvin, and their Churches did not follow Sola Scriptura in practice either. In order to learn, understand, and explain the Bible they and their Churches relied on the writings of the Church fathers, especially Augustine. So De Facto they were treating Augustine and other writers as authorities to understand the Bible's position on questions, as opposed to using only the Bible alone on them. Further, when it came to the practice of Church administration, they along with the Lutheran and Calvinist Churches found that they needed to use church leaders, documents, and institutions as authorities. So whereas the preceding generations of Christians used bishops, declarations, and councils like Nicea in addition to the Bible to decide teachings, Luther and other Protestants in practice did that too. The Lutheran and Calvinist bishops or elders, formulas or "confessions," and assemblies de facto were making decisions on major religious issues like whether to baptize infants. And their institutions had practical force because for instance they could depose Protestant clergy if those clergy violated the assemblies' decisions. In other words, it was not enough for those clergy to accept the Bible, but they also in practice had to accept the decisions of those Protestant bishops, elders, or assemblies. Calvinists could claim in response that they just have "elders" whose "confessions" just match the truth but are not "authoritative" like the bishops and canons of past Church generations who took decisions on issues. But in practice, they and past generations were both meeting the same definitions of "authorities" and "rules" because both the past bishops and the Calvinist "elders" had power to make decisions on issues.
Finally, Sola Scriptura is one answer and reaction against the Catholic idea of the infallible Magisterium, but is not actually the only or best answer to it. A more natural answer to the topic is the Orthodox Christian or Anglican or Methodist principle where Scripture is the highest authority, but not the only authority. In Anglicanism and the Methodist Church descended from Anglicanism, this principle is called Prima Scrptura, the idea that Scripture is the First in authority. The Methodists have Scripture as part of the Wesleyan Quadrilateral that includes Scripture, Tradition, Reason, and Experience.
To better understand the Sola Scriptura principle, it is best to see how it stands in an opposite tension or "dialectic" with the Catholic Magisterium concept. The Catholic Church developed the idea that if all bishops everywhere agree on a teaching, then it becomes part of an "infallible" Magisterium. Luther was probably a Catholic monk reacting against this Catholic concept in a kind of action-reaction "dialectic." He objected to numerous Catholic traditions, and so to give his objections power, he advocated the "counter-dogma" of following only the Bible alone, not traditions. In essence, Luther threw the baby out with the bathwater. While he was right to challenge many of the Catholic Church's decisions, much of extrabiblical Christian Tradition has key value for Christianity. Further, Luther's premise that Christian institutions or writings outside the Bible have no authority is not actually correct or practical, as Luther's own practice as a church leader showed.
Luther's theory may have missed the fact that just because something like a Christian council, declaration, commentary, or assembly might be mistaken or "fallible" does not actually mean that it has no authority or should have no role in rule-making. His theory may have also overlooked that even if one considers the Bible as the highest possible written source to decide on any issue, it doesn't follow that it is the only source. Nor does it follow that it is best or realistic to use the Bible as the only source. This is because while the Bible has general positions, like Jesus using wine at the Last Supper, Paul recommending alcohol as being healthy, and to avoid drunkenness, it doesn't have clear on-point directly relevant positions on every issue, like whether one can unconditionally only ever use fermented red wine for Communion. One can theorize that the Bible's story of the Last Supper implies that we should use fermented red wine unless such wine is unavailable, like if you are in the arctic wilderness with no grapes. That is the Orthodox Church's position. But the fact that the Bible is not on-point on the question is what helps give rise to variations like medieval Catholics giving only wafers or many Protestant congregations using grape juice.