I agree. it can go a few ways and most favor the Rams longer term.
He can play really, really well and earn a bigger deal. They could decide to give it and they keep a premier CB. Or they don't and they get a 3rd round comp. pick in 2018.
Like you said at the same time they further evaluate what they have in Gaines, Sensabaugh, Joyner, Roberson, etc. The only way they really lose leverage is if Johnson plays really well and the other four CBs don't.
Again in the end I think they have to decide between Johnson, Ogletree, Brockers, McDonald and Austin. Realistically they can probably keep three while having an eye on extending Donald (for an obscene amount) in 2018. This franchise tag gives them another year with all of them to make more informed decisions.