Why? Hadden only played half of last season. Slaughter was a liability when he played, which was rare. GJL was solid, but not athletic enough to play press man. Tank was terrible at taking angles and too slow. McDonald was meh. Wes Walker was hit and miss, but not SEC-level athletic.
We added Jermod McCoy, a freshman All American last year. Ricky Gibson is big and athletic and will play in the NFL. Prototypical CB. Jourdan Thomas gives Tennessee a huge athleticism upgrade at Star over McDonald. Jakobe Thomas > Tank. An improved Turrentine should be an upgrade over Walker (started to show that last year). Throw in Boo Carter, John Slaughter (probably the most athletic Safety on the team), and Jalen McMurray, and this group shapes up to be an improved group. Everyone who has seen practice has reported on the improvement in the secondary and its athleticism. This means the play calling can be more of what Banks wants to do.
Basically, we lost a bunch of guys who was average to mediocre SEC players (other than Hadden, who again, only played the first half of the season), and replace them with guys who fit the bill of being the kind of big athletic DBs required in the SEC.