That’s no question of belief or experience. It’s simple math. Martigale, d’Alembert or similar systems all reduce the profit in the long run.
Their theory is that your chance to win increases after you lost. But it won’t, on the contrary, strategy results are often autocorrelated, so after you lost a trade, there’s a better than random chance that you’ll also lose the next one. That means you lose more on average when you increase the trade size after a loss.
Extreme Martingale systems, such as doubling the size after a loss, always get your account wiped. This is mathematically unavoidable. The only question is how long it will take. It can be a year, or it can already happen when you start trading. Until that happens, your profits will be much better than without Martingale of course. So the longer it takes, the more you’ll lose.
Martingale systems have only two uses: When you want to sell a robot or EA, such a system can make the backtest look much better. And in some rare cases when strategy profits are strongly autocorrelated, an Anti-Martingale system can indeed increase the profit.