[gojomo]

In particular, if emin-gun-sirer had gotten his 2013-Nov-03 prediction right, and that was a good time to sell your Bitcoins, don't you think he'd be shouting it from the rooftops, ever since?

He took a gamble: that the bold tweeted prediction would increase attention for his paper (which it did), and then come true, improving his credibility about such matters (which it didn't).

He's now saying "I told you so", literally, in a gambit for more credibility – but eliding mention of his prior bad predictions. (In addition to the "good time to sell" prediction, I would point out that (a) orphan rates since his paper was published have not shown the predicted wide self-interested adoption of "selfish mining"; (b) the pool that's achieved 51% does not appear to have used "selfish mining" to get there - just the same economies-of-scale and small-miner-superstitions that have been known threats since the beginning of pooled-mining. In other words, even if this is a disaster, it's not the same one, by the same path, as he predicted – but an older potential doom, prophesied by others long before his work.)

When facing such a claim of earned authority, it's entirely appropriate, and not at all ad hominem, to highlight the full record. To "shout from the rooftops" exactly the same early calls he himself would be bragging about – if he'd been right.