It's true that one can do worse with DCA. But if stocks go up one makes money, just less. And if they go down one loses money, but less. In other words, one is more likely to get a return closer to the long term average, which is the objective.
If you've been averaging starting at the top of 2000 and putting the same amount of money in each month you'd be losing money right now 10 years later. You can say well I'm not retiring in 10 yrs and would be saving for 40 yrs. But that's 20% of the time which is a lost time opportunity of a decade.