You make some good points. However, it does take some time to get the personnel you need to play a particular defense.
So then the question becomes: How flexible should the team be in its defense? Should they change their defense each year to suit the talent that's available in the draft? That's the tough part. Right now the most recent trend seems to be toward the 3-4. A few years ago, the cover 2 was hot. And yet, one of the best defenses of this era is the Steelers defense, which is and always has been the 3-4. So which way do the Bills go? Change defenses every year or two to suit the draft talent available, or pick one scheme and stick to it?