I think it's an important thing to remember, during your search for answers to this question, that Young Earth Creationism in its present form is a relatively new entity.
While the concept of the Universe as young has been the dominant view throughout Christian history, that doesn't mean that a strictly literal interpretation of Genesis has been the sole view throughout much of history. Prior to the 18th century, there was no real reason to suspect an old age for the Earth. However, allegorical interpretations of the days in Genesis did exist as early as the fifth century, which is stronger evidence for such an interpretation than if such explanations had only begun to exist after scientific ideas suggesting an Earth much older than 6,000 years were formulated.
Through the Middle Ages, allegorical interpretations of Genesis were fairly common.
While I understand that you are Protestant and do not want to turn this into a debate over theological issues, it was with the Protestant Reformation and the advent of a sola scriptura rule of faith that literal interpretations of Genesis became dominant in Germanic and English-speaking parts of Europe.
With advances in science during the scientific revolution, a non-literal interpretation of Genesis was again suggested, although it took some time for the strength of scientific ideas to filter through to clergy and theologians. Early Young Earth Creationist groups, like the
Scriptural Geologists, were the first to suggest that sedimentary layers were put down by the Great Flood, having the peak of their support in the 19th century. At the time, they were still largely regarded as a fringe group. By the early 20th century, several different interpretations of Genesis had developed among all branches of Christianity to explain away the discrepancy found between science and a literal reading of the text. Evolutionary biology also existed as a field of research by that point, experiencing limited acceptance among conservative Protestants, some (gradually increasing) support among Catholics, and largely universal acceptance among more liberal Protestants. Despite this, however, Young Earth Creationism had almost no support among any group.
The authors of
The Fundamentals, the essential document of conservative Protestantism in the early 20th century, were able to confidently assert that virtually no one accepted a recent creation. A form of Young Earth Creationism may have been more common among smaller, more localized religious communities, but among those who left records discussing the subject, the assessment of the authors of
The Fundamentals seems accurate. Even during the so-called "Scopes Monkey Trial", William Jennings Bryan expressed a belief that the Earth was not created recently, despite his opposition to evolution, suggesting that the belief was not common among educated creationists at the time (I make reference to educated creationists not to be offensive; "un-educated" at the time often meant having no scientific literacy and very little actual literacy, so the opinions of non-educated creationists at the time are difficult to discern).
Seventh-Day Adventists, due to a perceived requirement for a belief in a literal seven day creation in their unique belief system, attempted to resurrect the beliefs of the 19th century scriptural geologists. A man named
George McReady Price wrote on the subject, and is pretty much the grandfather of modern Flood Geology.
His work was filtered through two men named Whitcomb and Morris in the middle of the 20th century, and reached a broader Evangelical Protestant audience. By this point, acceptance of an old Earth was essentially universal among Catholics, and acceptance of evolution had been explicitly permitted by the papal encyclical
Humani Generis, so reception of the book
The Genesis Flood was likely much less warm among Catholics.
The conservative Protestant view of science in general began to diverge substantially from mainline Protestants and Catholics at this point. Whereas before the publication of
The Genesis Flood an old Earth had been widely accepted and even theistic evolution was beginning to gain ground, this particular text spread widely among Protestants outside of academia, and started the acceptance of Young Earth Creationism essentially from the ground up. Basically every YEC ministry currently in existence has come into being since the time of its publication, and acceptance of its position has increased even among conservative Protestant theologians.
In my view, the fact that the modern movement started in the middle of the 20th century, propagated by two people who had very limited competence in fields related to the age of the Earth or evolution, should cause caution in accepting it as the only orthodox view on the issue. I would strongly suggest looking into the scientific problems with Young Earth Creationism from a serious perspective, and investigating alternative explanations for the discrepancies between a literal interpretation of Genesis and science (I personally hold to the
literary framework view). I think you'll find that Young Earth Creationism is neither a necessary nor a particularly ideal position to hold.