The returning starters on defense is certainly a flaw, but I feel that the other team's flaws will be greater.
In Spurrier's 3rd year and Meyer's 3rd year, we experienced a low point in talent due to attrition from the coaching changes. Saban was able to win in that 3rd year, but maybe you are correct in observing that their exodus came 1 year later.
However, the benefit of the way Alabama's big exodus wasn't until after the 3rd year means that they are replacing an entire front seven... but with juniors or seniors as projected starters at all 7 of those (at least according to the preseason mag I'm looking at).
The secondary shows one returning starter and 3 sophomore new starters, which could be a concern... but again, I'll take Alabama and these concerns to be the preseason SEC West favorite over the other teams and their respective concerns.
There are no clear cut national favorites, so I guess that's why the coaches poll has us at #3 and Bama at #1. While I am not saying that can't prove to be true, it is a little surprising, no?