You have Derek Carr making $25 mil. Would have been in the playoffs 1 time, but suffered a broken leg in the last game, but let's say he would have won that game, still that's no more than Dak has. ONE winning season in 5. Overall 32-46 record. Dak's been in the playoffs 2 of 3 years, overall 32-16.
Matthew Stafford makes $27 mil average, and he's been in the playoffs only 3 times in 9 years, lost every time.
Kirk Cousins makes $28 mil, and he has an overall losing record, lost the only playoff game he started in 5 years.
Is Dak "worth" $28-30 mil a year? I'd say no, $25 mil is more like it. But consider this - in May of 2018 Matt Ryan became the highest paid quarterback at $30 mil a year. 5 months later Rodgers got $33.5 mil a year. 8 months later Wilson gets $35 mil a year. Now Dak isn't in their league, at this point anyway, but as the top salaries go up, lesser players' salaries also go up, on the average. $30 mil now would be tied with Ryan for 3rd highest. But what about 1-2-3 years from now? The top salary could well be near $40 mil, like say if Garroppolo really shines, or Mayfield continues to impress, etc.
My point is that IF Dak improves, or even just the team improves, if the Cowboys wait then Dak could easily make a case for over $30 mil a year. Locking him in at even $30 mil now will be a great deal in a year or two or three. Of course he may not improve, but that's what the Cowboys need to decide - do they take the gamble, or not? If they tag him, then they're still out nearly $25 mil, not that much less than $30, and even closer if he signs for $28 mil.
It's an unknown, of course, if you think Dak has plateaued, then he's not worth $28-30. If you think he will improve, then it's pay him $28-30 now, or $32 mil next year...