Yes. Really that's the only reason. We know he can do it. We know he failed in Detroit when his team wasn't good enough, just like Dak has failed in Dallas, but we don't know if Dak would have had the same success in LA that Stafford did.
Stafford's record as a QB is 108-113-1. His career completion percentage is 63.4. He has a 4.6 TD percentage (377) to 2.3 int. percentage (188). And his career QB rate is 91.2.
Dak is 76-46; 66.8; 5.1 (213); 2.0 (82); and 98.1. Technically, he should be the better player, but Stafford has a ring. I do think given the right circumstances that Dak could get a ring, too, but I certainly don't have proof of that.