Only two of 11 incumbents in past years, Jimmy Carter in 1979 and Barack Obama in 2011, were behind at this point. They were down by 4 points and 1 point respectively to their eventual challengers (Ronald Reagan and Mitt Romney). Carter went to lose reelection. Obama went on to win with a small reelection margin — and there were many polls at this point that had him ahead. (Remember, I’m looking at the worst poll for past incumbents.)
Put another way, Trump’s worst poll against any of the top five Democrats at this point is 5 points worse than the worst poll for any incumbent since World War II against his eventual challenger. It’s 12 points worse against his most likely challenger, Biden.
As I’ve already mentioned, we don’t know if these polls will hold. What is notable, though, is that Trump is not punching above his approval rating right now. Trump’s approval rating has been consistently below his disapproval rating, just like he has been consistently been polling behind Biden. That lines up with what occurred in the 2018 midterms: Republican House candidates got the same share of the votes as Trump’s approval rating, 45%.