I doubt that Cain possesses the physical capabilities to be a closer. Granted, he'd be asked to throw only 20 or so pitches instead of 100. But closers must be ready to throw at maximum effort for 60 to 80 games, and I'm not sure if Cain's arm is resilient enough to handle that.
Suarez still has to win a job. He's not experienced or accomplished enough to be considered a lock. If Cain does not secure a spot in the Giants' 2017 starting rotation, it's easy to envision a scenario in which he becomes the long man and spot starter.