The mistake in 2016, IMO was a) the extrapolation that came from those polls and b) people paying way too much attention to national polls, which have very little connection to electoral outcomes, given the electoral college.
Also perhaps c) the larger public not “getting” statistics in the way they’ve been presented. The NYT had, if I recall, Clinton at 90% chance of winning. That still means that in one of every ten flips of a coin is a Trump win. But people read “90% chance” as “definite win”. I don’t actually know what anyone should or could do about that.