The way I look at it is they will have had Johnson for 6 years which is pretty good for a 3rd round pick. And he'll have made almost $35M from the Rams in those 6 seasons. I always have liked him and was in favor of keeping him over Jenkins. To me he was always the more consistent player and I grew tired of Jenkins' freelancing.
But you can only hand out so many huge contracts and I'd rather also be able to keep a few more mid-tier players. Donald will get the major contract soon. I'd rather retain Ogletree and either Alexander or Joyner beyond 2017 and let Johnson go. Hopefully he has a great year, earns a major contract elsewhere and the Rams get a top 100 pick in 2019 for him. Fair value.
It does, though, contribute to CB being probably their top need for 2018. Johnson, Gaines, Robey-Coleman are all UFAs after this year. So is Joyner (whether he's a FS, slot CB or both) and Alexander. Maybe a young CB like Jordan or Penton carves out a role but they're still looking at a major gap to fill for 2018. But with all their draft picks and plenty of cap room next year it's a position they can restock next spring.