I actually think they're in a good position with Johnson. Like you said his $16.7M is not holding up getting Sullivan, any other 2nd tier signings, or even extending Donald. They are just fine cap-wise for 2017 and especially 2018.
Now maybe they do trade Johnson at the draft. My guess is it would be for a Day 2 (2nd/3rd round) pick after a CB-hungry team misses out on someone in the 1st round.
But I can also see them just keeping Johnson this year as they eye all their 2018 cap room and then a 2019 comp. pick for him. Maybe they draft a CB at #37 anyway and go into the season with Johnson, Webster, Joyner, the 2nd round pick, Gaines, and Jordan/Hill battling for the 5th CB spot. Who would complain about that?