In a perfect world, where qb's didn't get hurt, having Mac Jones be able to run like Jackson would be awesome. However, it never seems to work that way. It is almost like to be a great nfl qb you have to be able to pass, and if you are an amazing runner, you have been trained since you were young to run as soon as things start to fall apart. So you get to the NFL, have some success early, then either realize you have to stay in the pocket to pass, or you get crunched 1 to many times and you lose a step or two and still can't pass in the pocket as well as a qb that has been doing it his entire life.
That of course doesn't mean that Jones is the next Brady, that the Patriots should draft him, or that he will be any good at all. What it does mean is discounting a qb because he can't run is as foolish as saying all qb's that come from Alabama suck.