It is the scheme. Everyone likes to say that the players suck, but at the NFL level, nobody sucks. It's just about using the personnel correctly to maximize their strengths and minimize their weaknesses.
The FO's mistake at safety was having two clones in Bell and Wilson. They are basically the same player - both excellent at stopping the run, coverage skills and ball-hawking lacking.
At LB, our guys are just not that good in coverage. When you look for big guys that stuff the run, you're going to end up with guys that can't run with speedier or more athletic TEs. A safety who is good in pass coverage could mask this.
Now I am sure there are defensive schemes that could be used to better mask these deficiencies...perhaps more of a zone-based defense. We switch to a zone occasionally, but we don't do it well. Our defensive philosophy is a man-to-man win your battles kind of thing. We don't have LBs or safeties that can really consistently win those battles, but we aren't about to change our core philosophies on D either.