"How do you know Tom Brady won't blow out a knee"?
"How do you know that Russell Wilson won't finally have the injury from running around, that so many have predicted"?
"How do you know that after losing so many players to FA that the Eagles can be anywhere near as good as last year"?
How do you know that the Vikings won't actually regress with the signing of Kirk Cousins"?

I could go on and on.
These sorts of doubts aren't unreasonable necessarily. But they are completely unanswerable and predict the worst possible outcomes.