I kind of disagree. In primaries there is less and less reliable polling so that approach is a lot less valid. What he did was to look at historical numbers with predictive value - like endorsement points and such. And based on that he was willing to distrust the polls.
Obviously, this was wrong, in hindsight. But I don't think it was as dumb or ludicrous as people make out now.
Not being sure means putting big error bars on predictions, not claiming that one outcome is super likely.
Did you read the article? It cites a fair bit of data in arguing that polls at that stage has weak predictive power. This is undeniably true. It then says:
"So, could Trump win? We confront two stubborn facts: first, that nobody remotely like Trump has won a major-party nomination in the modern era.4 And second, as is always a problem in analysis of presidential campaigns, we don’t have all that many data points, so unprecedented events can occur with some regularity. For my money, that adds up to Trump’s chances being higher than 0 but (considerably) less than 20 percent. Your mileage may vary. But you probably shouldn’t rely solely on the polls to make your case; it’s still too soon for that."
I don't see that as being insane, dumb, ridiculous or even particularly wrong. People keep saying that Silver shouldn't have tried to be a pundit, but it seems to me that predicting Trump was a lot about punditry. Primaries don't have a lot of reliable data and what we know of historical data all pointed away from Trump. Making a wrong prediction doesn't make someone an idiot, it's the quality of the analysis leading to the prediction that matters.
