That is another conversation for another day when that time comes. But, if you want to take this debate to hypothetical land, I'll walk with you for a bit. Just keep in mind, your OP was only comparing Zeke vs Miller + the Cowboys pick of what was left in the draft. I stand firm in my belief that the Cowboys made the right decision...in fact, I predicted it.
The first thing you must consider when looking 5 years into the future is that the Cowboys team will likely no longer feature Romo...or Jerry Jones, given his age. So, by merit of how difficult it is going to be to find Romo's successor, the Cowboys are in win-now mode. 5 years from now Dez will be a shell of his former self. Gone will be Witten, Sean Lee (probably) as well as a host of other players on the wrong side of their prime today. That is the nature of the business. The truth is if you have a good young quarterback, you can afford to make more picks with the longer-view in mind; you have that luxury...the Cowboys don't.
Also, given the strength of the Cowboys line as it stands, there is a good chance they are going to have say goodbye to a very good player at some point in the next 5 years. Once again, that is the nature of the business. The truth is, in total, about half of the Cowboys roster will say goodbye in the next 5 years.
My point is, if Zeke is still productive and the Cowboys are in a rare window to win and possibly compete for a championship, there is great chance they will make a play to keep him. If they are not in that mode, they may say goodbye, but at the point, the fans who understand all of the above will not really care to keep window dressing on a shanty.
So again, regardless of how you slice it, the Cowboys made the right move. Some, however, will have to see for themselves what Zeke adds to believe it.