Correction: “They *had* the best offensive line in the league.”
Several of last years O-line players are injured and a few have left the team. The replacements are not as good. And the defensive secondary is mediocre at best.
Also the Cowboys are very overrated. They had a poor year in 2015 which set them up to play the dregs of the No Fathers League in 2016. They had a great year in 2016 by basically picking off the low-hanging fruit and now, without any significant upgrade in personnel, they are playing the better teams. The Cowboys *may* get to 8-8 but they will be watching the playoffs from their sofa.
The Eagles are in the same scenario albeit one year later. They are doing great but are not playing the cream of the league because of their previous year. They will probably win the NFC but expect them to go 8-8 next year.
With Da Iggles, the best you can hope for is one good season per decade, if that.