Plug the numbers into the Kelly criterion.

The fraction of your wealth you should bet when the coin is fair and you get paid evens is zero.

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.

And yet in the history of the world, countries and economies get destroyed all the time.

True. But if the US equities market fails, there will be more to worry about than your stock portfolio.