Dalton completed about 65% of his passes, extrapolating his 9 game numbers to a full season would give him 3,857 yards, 25 tds and 14 ints. And he played most of his games without Martin as well as all without Smith.
But considering he just got to the team last year, had no preseason work and little camp to speak of, with more time with the receivers and in the offense it would be reasonable to assume he'd do more like 4,250 yards, 30 tds and 18 ints. No great numbers, but not bad, either.
So the Cowboys would probably be "competitive" with Dalton (assuming a 16 game availability) Gilbert didn't do all that bad in his one game, obviously needs game time experience. And if the Cowboys would let Dak walk, sign Dalton as a "stop gap", and you have Gilbert to "develop"
So there would be a plan b if Dak left. Not that I'd like to see that, but just noting it's not like the Cowboys don't have options..