[notahacker]

I don't think the ratio of failed predictions is the correct basis for assessing the quality of the prediction, particularly not with something the predictor himself explicitly states is a low probability event. If I only give a handful of horse-racing tips but one of the long shots comes in leading to a high ROI, your prior for me just being lucky should still be high. Especially if none of the factors I've suggested which might make that horse more likely to succeed than predicted come to play, but it incidentally wins the race thanks to a massive pileup that takes out most of the other horses.

It's that second bit which really matters. Sure, if you had taken gwern's advice to mine a few BTC in 2009 and sold in late 2017 you would indeed have made the $10,000 per BTC mined hinted at. So it's easy to take that as confirmation of the quality of his reasoning.

But what gwern actually says is that BTC has a +ev on the assumption that it has up to a 0.1% chance of "eventually replacing a decent-sized fiat currency" and in order to do so, the value of an individual BTC would need to hit $10k. This replacement of "a decent-sized fiat currency" hasn't happened. The status of the prediction that BTC had a low but not trivially low chance of replacing "a decent sized" mainstream currency is indeterminate.

It's entirely plausible that when deciding whether or not mining was worth a bit of time and electricity gwern also weighed up the possibility that BTC would have great potential as a speculative asset due to an ideologically committed base who would want to hold it, public interest due to novelty and hype, and readily manipulable markets, but that's not in the post. This might even mean that he considered the situation that has come to pass as less likely than a scenario which hasn't. (Maybe he'll be along to tell us what he thought about that at the time!)

From the point of view of people who listened to gwern being richer than people who thought he was wrong, that might be an irrelevant detail. But ignoring the possibility the reasoning about the probability of x being systematically underestimated by markets might be entirely wrong because x occurred anyway is also a form of confirmation bias.