The other major factor playing against Mack is the franchise number itself. For some reason, the collective bargaining agreement in the NFL lumps all offensive line positions together when it comes to the franchise tag.
Thus, if a team wants to franchise a center like Mack, they have to pay him like a left tackle. As you can see in the chart above, that would put him somewhere around $11.1 million for next season. When compared to centers on the best lines in the NFL, that is an astronomical amount.
If Mack is on the Browns next season, it will be because the front office re-signed him to a long-term contract and not because he received the franchise tag. To put it in perspective, New York jets center Nick Mangold is the highest paid center in the NFL at $6.5 million per year.