Don't get me wrong, I think Dak is an above-average QB who folds under pressure. That being said, there's no guarantee that the Cowboys will be successful if they move on from Dak. But, they haven't won anything major with him behind center, so why not go in a different direction? My point is, if the Cowboys are going to just let Dak walk, they should at least try to get something for him and have a plan in place for when it happens. My other point is, I don't believe his asking price is going to be conducive to putting a SB team around him. That's just my opinion. Your opinion may be different.
Now, to answer your question. The Niners brought in Young while they still had Montana. (Even though that was a trade) The Packers brought in Rodgers while still having Favre. The Patriots drafted Brady while they still had Bledsoe. The Ravens brought in Jackson while they still had Flacco. Those are just a couple off the top of my head. But, once again, there are no guarantees in football.