The defense won't be fixed in a year's time, no. But locking up Cutler means looking past next season. To keep Cutler long term and make the defense good again, they're going to have to eventually cut loose at least some of the weapons that Cutler has right now.
The offensive line was, why can't the defense? The defense was killed by injuries this year. Healthy players + a free agent or 2 + more experienced young players + draft, all could lead to a much, much better defense next year (can't get much worse). 2 years ago, the defense was good...losing Urlacher didn't make them go from a top 10 defense to the worst.