While I agree 90 % with you, I would say that it´s not totally ahistorical. Sola scriptura was not exclusively Luther`s fetish. (Melanchthon was the main guy behind this idea, but there would be tons of others.) In the early 16th century, you had a) the printing press and b) humanistic discourse for a century or so. While the latter stressed the relevance of philology, of serious engagement with old texts in general (ad fontes) long before Protestantism applied this to the Bible, the former provided for the first time the technological foundation for spreading ideas and arguing beyond elite circles.The problem with this is the post-Luther assumption that its being biblical or not mattered to many people at the time. The whole "biblical" thing is a Reformation meme (sola scriptura). Both the Eastern and Western churches always have based their creed and institutions ultimately on a direct line of succession from the Apostles, on Tradition, and custom (the papacy, by the way, is based on the historical St Peter's role and presence in Rome as bishop, but was also supported by the passage in the Gospel of St Matthew); until the various Protestant figures came around, the idea of a requirement for every teaching of Christianity to be based on a sort of biblical appeal or footnote would have been entirely foreign. It's simply an ahistorical concern.
Besides, there is some basis for these teachings that you mention (purgatory, vicarious satisfaction, saints, etc.) in the book of Maccabees, which was removed from the standard Protestant biblical canon by... guess who.
A reformation in the 16th century was not altogether that unlikely to bring forth sola scriptura or at least to be more biblicist than the late medieval theological mainstream. Society had changed by that point. That is the big difference from the situation one or two centuries earlier.