This is the easy way of looking at the roster from year to year:
The previous year's roster, minus free agency losses, plus FA additions, minus a few guys who "clearly" suck...equals some number of spots that need to be filled. So you just draft those guys and you're done.
The reality is: This is a method on how teams get worse in the NFL.
If a you want to get better, you look to upgrade all over. Now, will you upgrade at every position? Of course not. There aren't many/any upgrades over Colin Kaepernick, Mike Iupati, Patrick Willis, etc. But what about others? Maybe we just shrug our shoulders and say, "Well, the guys we have in this position group form a decent-enough corps, so there's no need to add players there."
The pass rush, for instance, was good at times and absent at others. When Justin Smith got injured, Ricky Jean Francois filled his role. The pass rush completely fell off. If there were better options, they would have played. There were guys who had "DT" next to their name, but clearly Vic Fangio and Jim Tomsula felt these players would not make a huge impact, and so they continued to sit. What if there were a young stud or two who could not only serve as a disruptive backup, but could even rotate in and give the starters a breather, perhaps preventing an injury or a drop in effectiveness due to exhaustion?
What about the secondary? Let's say Nnamdi Asomugha, Carlos Rogers, Tarell Brown and Chris Culliver all stay on the roster. Is this the best four guys the team can get? Or perhaps we're saying, "Well, last year we were this close, so we just need to do a few little things, then we're good."
I think you need to let go of all restrictions when it comes to the draft and player additions in general. Forget who's on the roster already or who could be ready to take on a larger role. Stop thinking about what the team doesn't need, and just add talent.
Get the best guys you can at positions where you don't have All Pro's; where you might have guys in the final years of their contracts or where the depth is such that the starters play every snap on defense. Put together the best 90 guys you can going into camp, roll out the ball, and let them compete.
What happens when you go grocery shopping at the convenience store? The beverages are top-name brands, there's some canned food and some milk, but you ain't gonna find steak or fresh veggies, etc. At best you'll walk out with something you can call "food"; but your choices were limited, and it's not a complete meal of the highest quality products.
Why limit the players you have to choose from by being hell bent on keeping the same guys, simply because they were here last year? Who knows if some of these guys will get injured, are slowing down due to age, aren't as motivated or are distracted by something this year? Maybe this is the year they fall off the face of the Earth, as this happens to so many players in the NFL.
What if that rookie CB you bring in outplays everyone on the squad? You look back mid-season and the guy has six INTs and nobody can complete a pass against his man ... Still upset that we cut or traded Tarell Brown? It's possible, but you'll never know if you refuse to consider the possibilities.
I think there will be much more turnover than people think. Draw up the starters, then remove the players who most likely could not be improved upon, as well as the guys you just couldn't live without. The "maybe" guys who remain need competition and should possibly be upgraded. THAT's how you get better.
Everyone else is putting together the best roster they can. You can't be as good as you were last year; Your team must keep up with the rest of the league, as they're not sitting around doing nothing.
I could see three quarters of the secondary changing, including backups and depth players. I could see most of the reserve defensive linemen gone, replaced by top-level draft picks (first three or four rounds). I could see a shake-up at the WR position, with perhaps just two or three guys from last year remaining.
I like these guys; the depth we have. I want to believe they're ready to take the next step, and the team does, too. But they're not going to pass up the opportunity to add quality talent at most positions. The best 53 make the squad. Why not start with the best 90 to get there?