[RussianCow]

Nothing is "100% guaranteed". There are lots of reasons that the returns of investing in index funds using DCA may dry up, or even fall over the long run—for instance, it's not inconceivable that a significant enough market downturn could bankrupt a lot of Fortune 500 companies, bringing the indexes down significantly for a long period of time. Even though these types of scenarios seem extremely unlikely, it's incredibly inaccurate to say that any such returns are "100% guaranteed".

[jiofih]

Yes, you can argue that, but it’s statistics, and includes the three big crashes the market has lived through. It’s a 20 year period for a reason - it’s how long it would have taken you to recover from 1929 if you bought at the peak just before the crash.

[RussianCow]

> It’s a 20 year period for a reason - it’s how long it would have taken you to recover from 1929 if you bought at the peak just before the crash.

You've cherry-picked an arbitrary window based on historical returns, but there is no reason that this will hold in the future. You can come up with lots of strategies with arbitrary conditions that would have given you great returns throughout the past century, but completely fail after you implement them. I don't see why dollar cost averaging is any different.

> it’s statistics

Exactly. And nothing in statistics is guaranteed. The fact that it's worked well 100% of the time in the past does not imply that it will work 100% of the time in the future. Does it seem highly unlikely that DCA will stop being an effective investment strategy in the foreseeable future? Yes. But I take issue with the "100% guaranteed" claim, and everyone should realize that no investment strategy is completely risk-free.

[jiofih]

Well, down here in the real world, a 100% statistical chance is as good as it gets. Why would you try any other strategy with historical success < 100%, or more likely, unknown?

[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.