Yeah I don't buy into Johnson starting ahead of Alexander. For one Alexander is going to be an effective SS and I think Joyner is the guy that will direct traffic from FS. But I do see them weaving in Johnson at FS situationally to get his feet wet. Probably in 6 DB packages where Joyner moves up to play CB again. I'd like to see Johnson ideally get 150-200 snaps this year as a sub and starting next year. Just my opinion.
By the way remember that 5 of the Rams top 7 DBs are UFAs next year- Johnson, Joyner, Alexander, Gaines and Robey-Coleman. My guess would be 2 of those 5 are kept and a CB is a high priority in the 2018 Draft.