I'm not sure how to answer your question: So why do Protestants treat his un-Scriptural teaching of "the Bible alone" as infallible?
As an ex-Lutheran, I came to the conclusion that most Protestants are nothing more than cafeteria Catholics. They are just inserting dogmas of their own to compete with Catholic dogmas which they find so objectionable.
If God has a favorite brand of Christianity then there are only three viable candidate groups:
There are very few remaining in group #3 other than a small Foursquare Baptist sect which I once attended who claimed to be the true followers of Jan Hus and considered mainline Baptists as sellout for accepting the authority of the Council of Nicea.
There may be other small independent sects with similar beliefs, but the vast majority of other Protestants put themselves squarely in the "cafeteria Catholic" category by accepting the Council of Nicea while rejecting the authority of the church which organized it.
Just the way I see it . . .
Thanks! I can’t argue with anything you said. The logic seems pretty straightforward.