I can just as easily say Trump would have beaten Biden easily. I think Biden is a lousy campaigner based on his previous attempts at the nominee. There's no way for either of us to know.
And while Comey may have cost Hillary the election based on changes in the polling, it's difficult to say for sure especially since it's the Electoral College and not the national vote that matters. She might have still lost the midwest states that she needed.
And you're assuming that Biden would have done better than Hillary.
I think it's mainly name recognition and pre-Trump nostalgia. Maybe Biden can maintain that lead, but I have doubts. Everything from his creepy touching, opposition to integrated schools, writing the crime bill, racist comments about Mexicans and building a wall, negotiating to cut Social Security - will eventually get out and we'll see how his numbers fair then.