Will the poll numbers be just as relevant in 2020 as those during the 2016 election where every single pollster except one predicted Trump's loss?

GBA has cited the only important Presidential poll.  They only do it every four years, but it is the poll that counts.

The pollsters' failure to predict didn't surprise me; they have a poor record on actual elections; which seems to be worsening as land lines disappear.

The London bookies even blew this one and they are usually much more accurate than our 'professional' pollsters!  They lost hundreds of millions!  The UK bettors knew more than our pollsters and made money proving it!