How long will this myth be perpetuated?
By the time of the election, Hillary was only leading nationally by about 2-3%. And guess what, that's also by how much she won the popular vote. And bar one, all of the swing states that Trump won he was also very competitive or even straight-up leading in those states.
Biden's lead over Trump is very different to the one of Hillary had over Trump. Not only is it larger in general, Trump is being decisively beaten in a number of states he carried 4 years ago.
Also, trump retracting ads from swing states is never a good sign:
People are very results oriented. They tend to focus on correct and incorrect rather than how correct one was or why.
It's kind of like a coworker who used to argue that Eli Manning was better than Peyton Manning because he had more superbowl wins (before Peyton won his second which was actually won despite his terrible play).
The polls in Texas actually were further off than the polls in Pennsylvania. Yet, nobody really cared because the end result was correct.
The polling data was undeniably wrong in the rust belt in 2016, but that doesn't mean polls in 2020 become completely worthless. We have to look at why they were wrong. And we have a pretty good idea. Lots of undecided voters, bad last minute news for Clinton, higher turnout among non-college educated whites, and good old fashioned margin of error. Looking at these factors we can tell that a similar error in 2020 is pretty unlikely, since pollsters have adjusted their models, Trump is not winning by as much among whites without degrees, and voters seem much more likely to have locked in their votes.
To the extent that polling numbers are off, it's more likely to be due to mail in voting. That's not to imply anything nefarious will go on, but it could mean turnout numbers will be wrong. This could be in the favor of Biden if greater awareness, use, and accessibility of mail in ballots leads to more voters. It could also cut the other way. If a large number of mail in votes are not counted due to being filled out or mailed improperly, it could help Trump.
There is also the issue of ballots being counted in time. Democrats are pushing to extend the amount of time (which seems logical in light of a global pandemic) whereas Republicans are trying to deny that.