I think I'm almost with BL on this one. Koufax was basically an average pitcher until he suddenly wasn't for the last 5 seasons of his career. In those seasons, he went 111-34 with a 1.95 ERA, 167 ERA+, and 9.4 K/9 and won 3 Cy Youngs. Over the past 5 seasons Kershaw has gone 88-33 with a 2.11 ERA, 172 ERA+, and 10 K/9 and won 3 Cy Youngs. That's basically a wash. And while the volume is lower, Kershaw's achievement before the last 5 years (and so far this year) was at a much higher level than what Koufax did before he peaked. And of course, as of now, Kershaw is having the best SP season in history in 2016.
I don't even think he needs another 5 good seasons. I think if he drags himself through 3 or 4 more years of utter mediocrity he cruises in on the first ballot. But that seems highly unlikely.