The beauty of Matt Harvey, maybe above all else for the Mets, is that he isn't going anywhere. He can't leave them for another team next winter, the way Jose Reyes did in 2011. And they're not about to trade him, like they did with R.A. Dickey in 2012. He is, at present, their most captivating player. And he can be theirs at least until the end of 2018, when he is due to become a free agent.

But in baseball, letting a young, homegrown star reach free agency has gone the way of rotary phones and true doubleheaders. The prevailing...