In the era of central bank's helicopter money, the market will always be skyrocketing up and up given enough time.
What's the strategy to profit from indices?
Only short the market when its in a state of euphoria /irrational exuberance bubble, or sell when it is confirmed (20% drawdown). Otherwise, you really have no reason not to long at every chance.
WARNING:Martingale is susceptible to huge drawdown spikes, use at your own risk.
simple functions for martingale wins and losses, multiplier can be adjusted manually to increase/decrease performance/drawdown.