Also, saying that the Eagles offense was the 3rd best in football last season is far from accurate. As a team, they did score the 3rd most points in the league, it's true. But they had 11 defensive and special teams TDs which grossly inflates their point total.
To get a more accurate measure of their offensive output, we need to look at another stat more representative of offensive success, such as percentage of offensive drives ending in points. In that metric, they ranked 12th in the league last season. Still good (especially considering what we had to work with at the QB position) but certainly not elite.
So they were a slightly above-average offensive team last season. What makes one think they can be more than that? Bradford is probably a slight upgrade over Foles, but he has to stay healthy for it to matter. Otherwise we're stuck with Sanchez again. DeMarco Murray, same story. Meanwhile, the WR corp has been downgraded, although there is a lot to like there long term. And the offensive line is a huge question mark, not to mention one of the oldest in the league.
I'm much more confident about the defense being good than the offense this upcoming season, which says a lot.