I think it is sustainable but a few things need to happen imo.
(1) Dak needs to improve.
Dak needs to become a better pocket passer and i'm fully convinced now that he will and can. The disappointing thing to me is his foot mechanics still seem horrible when even slightly pressured which makes his accuracy suffer. Hard to understand how this is not fixed after two and a half seasons. That alone will help alot.
His reads need to improve but that will happen with time and exposure. As much as Dak gets blasted here, folks seem to not realize Romo had the same problems the first half of his career. He missed reads, took too long in the pocket. Same stuff. Romo was more accurate and had better footwork but tried to look for the big play too much (nearly never taking the check-down underneath stuff), which forced alot of unneeded mistakes (often too much to overcome)
Dak on the other hand has taken the underneath stuff alot, but until recently has failed to push the ball down the field with certain looks (outside his first year). The good thing is,a he has been as of late, and largely accurately. If that continues, it's a good thing because that will help him continue not giving the ball up (which alone puts us in good position to win ganes), while at the same time not allowing the defense to cheat up..
(2) The O-line needs to have consistency.
(3) Someone is going to have to step up on the outside at wide receiver if they start taking away Beasely again. My money is on Gallup from what I have observed. Not sure if he is "ready", but he appears to me to be the only guy that can win contested balls, or Butler if he gets in the field.
(4) Yes I think they need to keep utilizing Dak's legs, but regardless.. that (1) thing still needs to happen. Defenses are at times going to take that away as well.