I think the Mets should try to talk Johan into it. I don't doubt he'd be a dominant closer and it would reduce his innings and protect and save his arm. I think it benefits Johan because it prolongs his career, and although he'd be making less money as a reliever, he would have more years in the league with reduced innings.
As far as it being dangerous for Johan to "get warm" or pitching every 3 days, I don't think it's an issue. He can adjust to that over time.
The first 32 million dollar closer!
But I think he will be a good starter this year. No worries. Once his ankle betrayed him, he relied more on upper-body, depleting his shoulder.
Plus, he spent all of last offseason preparing for season. Rest this year will be ideal for him. He won't reach velocity levels again but he has enough expertise to be effective, maybe even good.