When I get a new optic, I zero it at 7yds (kind of dictated by my local range, indoor, with 15 yd max lines). I have found the zero at 7 yds works very well at 3yds and 15 yds (the other two “marked distances”), and even 5 yds and 10 yds, which I have to estimate between the three marked distances.
As OP mentioned, I’m pretty sure a realistic self defense distance will be closer than 21 feet (7 yds).
The dots, to me, aren’t really about tight groups, they are about round on target as soon as possible. And to be honest, I always maintain that an instantaneous self defense employment of a firearm is going to be point shooting, likely using no sights.
Now if I was trying to line up a longer shot, say beyond 10 - 15 yds to maybe take out an active shooter like the mall shooter a while back, I’ll take a DOT over irons any day - mainly because I wear trifocals, have a lifelong astigmatism, and my eyes are just about 67 years old. In other words, I would have a hard time focusing on a front sight focused sight picture and reliably hit a target at distance. An optic makes that shot way more easier, and accurate, “for me”.
I do agree that more practice helps with dots, as does learning how to set your zero and figuring out a distance for zero that will work best for your predicted self defense scenarios.
But nothing says you can’t have/use/practice with irons and optics. I do it all the time, including some occasional point shooting without using any sights.
And if an optic fails at a critical juncture, you’re either going to be using irons or point shooting anyway, so it seems beneficial to practice those skills.