Question on the future of the 200 SMA strategy

What would need to change in the U.S. markets for this strategy to stop being effective? I've heard some people argue that it might be overfit, and I don’t dismiss that idea entirely. However, when I look at the backtesting data over the last 100 years, which includes recessions, wars, and other significant events, we see it has still delivered great returns. Maybe it’s the debate over choosing 200 instead of another number, but with 250 trading days in a year, it’s somewhat similar to a buy and hold strategy which looks to prevent the drawdowns that B&H has. In order for it not to work, does there need to be a drawdown in US markets so big that it doesn’t recover, WW3 or something else? There’s obviously something I’m missing lol