“Monmouth results in 2016 make their polls credible.”
Clinton Maintains Lead; Senate Race All Tied Up
Tuesday, Oct. 4, 2016
West Long Branch, NJ – Pennsylvania is looking good for Hillary Clinton, driven by gains among white women voters, according to the Monmouth University Poll . She currently has a 10 point lead over Donald Trump
Monmouth.edu
How do we know that this poll wasn't correct on 4 Oct 2016. Of course, we cannot know, this was a month away from Election Day and most data suggest that Trump made up considerable ground in October. Additionally, we now know that in 2016 a significant number of people lied to the pollsters.
I suspect this will be repeating in 2020. You make a good point, however. Watching an attaching any value to polls is probably a mistake.