Don't let fear or need dictate value. Let performance dictate value. Re-sign Joyner and Robey-Coleman in the secondary and still have money left over (vs. what Johnson is asking) to add a 2nd tier CB while also drafting one in the 1st or 3rd rounds. Short-term do you take a step back at CB? Maybe. But you've also solidified the secondary by then having your starting safeties locked up for the next three years. And as a nickel CB Robey-Coleman is basically a starter and he's very good in the slot.
And while you may take a step back at CB for 2018 take a step forward at OLB and in the pass rush, front seven. The $16M+ it would take to keep Quinn and Barwin could go elsewhere and enhance the OLB group with Longacre and Ebukam.