> The only predictions (both positive and negative) that I've seen have come from dogmatism and not from data.
Well, that's because we've never had a cryptocurrency of this scale before. Of course, we've had other alternate currencies (Ithaca Hours, anyone), and based on that data, Bitcoin's future doesn't look too bright? But you could make the argument that those aren't the same.
One reason that you aren't seeing much "hard" data in favor of Bitcoin is that the Austrian school of economics (which provides the foundation for deflationary economics) actively rejects empirical research and deductive reasoning[0][1].
In contrast, virtually all economic models that are taken seriously[2] do try to incorporate fundamental principles (inductive reasoning), but revise those in the face of empirical evidence when necessary (a process that's not dissimilar to what is done in, eg., quantum mechanics - a field in which theory oftentimes advances beyond the means to test theories empirically).
[0] https://en.wikipedia.org/wiki/Austrian_School
[1] https://mises.org/etexts/austrian.asp
[2] Yes, I am excluding Austrian economics, because very few non-Austrian economists actually take them seriously