[RussianCow]

I have this strategy where I put all of my money into the stock market on the 32nd year of the century, then sell it all off 10 years later. So far it's had huge returns 100% of the time. Is it a good strategy?

Obviously this is a wacky example and I'm trying to make a point, but you could overfit lots of different investment strategies that would have been 100% successful in the past; that doesn't mean they are guaranteed to work in the future.

And for the record, I'm not advocating for any other investment strategy. I'm just pointing out that you cannot possibly claim that any strategy is going to be 100% effective in the future, because you can't predict everything that will happen. The conditions under which DCA is effective may not hold forever, and no investment fully protects you from all risks.