Other than Power, I think Johnson is the only waiver exempt near-NHL ready defenseman in the organization. I guess a lot depends on any upcoming trades, etc., as to whether the team tries to slide any other D thru waivers, but once the season starts I could definitely see Johnson getting some short term callups for road trips, etc.
And eventually I think he'll stick. IMO he's by far the most talented of our options for 7th defenseman. He just needs a bit of time in Rochester.
Much depends on what happens with trades and waivers.