They can cover everyone. That's the advantage of having Earl Thomas. You run single high safety with Sherman and Maxwell or Browner on Decker and Thomas. The vertical game is completely shot. You keep the offense in front of you. That leaves Thurmond on Welker, or Browner.

Having ET back lets Chancellor play in the box. That kills TE's.
Greg Olsen - 10 targets, 5 catches, 56 yards
Vernon Davis - 5 targets, 3 catches, 20 yards
Vance MacDonald - 2 targets, 1 catch, 19 yards
Clay Harbour - 1 target, 0 catches, 0 yards
Owen Daniels - 11 targets, 6 catches, 72 yards
TE's aren't weapons against Seattle. The only one who did well received a heap of targets, and were therefore inefficient.
Peyton Manning will hit any open guy, but there are no open guys on the field. It becomes a check down game. RB's are averaging 3.3 YPC against Seattle this season.
Bend but don't break, keep the ball in front of you, and win the turnover battle. That's the 4-3 under. The way Carroll schemes his DB's is beyond ridiculous and he is the greatest defensive coach in the NFL right now.