It doesn't need to go to zero to make sense. The US market fell by a third in a single month at the start of the pandemic, recovered over the next five months on its way to new heights heading into this year, and now is down a fifth again. DCAing is simply a low-rent hedge against getting caught out by those kinds of swings. You won't get to gloat about going all in at the bottom, but also won't have to cry about doing it at the top.

DCAing is worse on average for simple objective functions, absolutely. Markets rise more consistently than they fall. But markets also fall faster than they rise. This may not matter to the hypothetical average person, but it matters to real people, and some of them make the perfectly rational decision, based on real-life economic factors that aren't captured by simple models, to trade returns for stability.

You can formulate this as e.g. an MINLP model and it isn't at all hard to construct situations that are favorable to DCA when lump-sum is an option. All it takes is adding constraints to reflect a real person's life circumstances. Other strategies are still better than DCA, but they're also more complicated to execute, and we aren't talking about sophisticated investors here.