Generally, teams don't want to pay any one player $30-plus million for a season. That's supposed to be the incentive for teams to get a long-term deal done. The player signing the franchise tag usually puts the pressure on the team because he's guaranteed now to make $30-plus million and then be a free agent unless the team wants to tag him a second year at an even higher price.
As I said in another post, I'd prefer for Dallas to play him on the tag and let McCarthy evaluate whether he thinks he's worth paying top-five QB money. However, I still expect the team to get a long-term deal done before July 15 because that's usually how it works.