Kelly criterion is applicable to betting / trading, where you're winning / losing. If you never 'lose', ie., stay in the market, martingale seems really promising. For instance, SPY will never go to 0. The FED / US govt will never let it. In fact, it can't - it would destroy the US economy. So when you're playing a game where you know the graph will always to up and to the right, eventually, martingale seems appropriate.

FWIW, I trade futures on the side and have always been interested in DCA or martingale long strategies.