So here's a joke that comments on the matter:
There was this helicopter with a pilot and passenger on board and they were lost in the fog somewhere around Seattle. The GPS was out, and so was the radio, and they had no idea where they were. Then up out of the fog loomed a tall city building. They flew in close and hovered and they could see people inside. The pilot told the passenger to write up a sign to ask "WHERE ARE WE?" and hold it up so the people in the building could see it.
The people inside the building squinted at the sign, and then hurried around to make up a sign of their own. They wrote it out and held it up to the window: "YOU'RE IN A HELICOPTER."
The pilot gave them the thumbs up and flew right off -- straight to Seattle Airport, without any trouble at all.
As they walked away from the copter, the passenger turned to pilot and said, "How on earth did that tell you where we were?"
"Simple," said the pilot. "The answer was accurate but not helpful... so that must have been the Microsoft Tech Support building in Redmond!"
The idea that any forum, but especially KB, has a "survivor bias" is very much like being told we're in a helicopter. It's true of every club, every institution, every organization, every forum. And it is most especially true of the profession of writing. (Or art, or entrepreneurship, or restauranteur.)
The sorting out of members of any group is so basic to the concept of entrepreneurship, you can hardly call it a bias. It's actually a correction against a bad data set in the first place: For instance, the life span of some insects, or maybe even sea turtles. If they survive infancy, they have a certain measurable lifespan. But if you include all the eggs laid, or even hatched, that drags the actual average down to a low number which doesn't actually reflect the normal life-cycle, in that the vast majority of the species either dies much earlier, or lives much longer.
The idea that successful people speak up more is actually a different bias. (And it isn't just about success. People who feel they've been done wrong also tend to speak up more -- so there is a bias toward conspiracy theories too.)
I've been in a lot of groups where the success/complaint bias has been extremely strong, and you can usually demonstrate it with polls and such. But I've never been able to catch this group in the level of bias you find in others. That's partly because self-publishing is more complex than some of the other places I've been, but it's also because it seems like HERE, people speak up a little more.
Not a lot, often just a little whine "I've only had four sales this month!" or something like that. And nobody here treats a person who says that like they're unusual. People make suggestions if asked, or they commiserate. As often as not, you get other people saying (without giving numbers) "My sales are down too." (While others say "Hey, MY sales are way up!")
What I like, though, is that there is a large variety of "success" levels among people who actually report things here. And people talk to each other in private too. This ISN'T a place where people go around setting specific goal posts, like saying that if you have X number of books you should be making XXXX amount per month. Everybody here knows and says often; EVERYTHING VARIES.
So saying that this group demonstrates "survivor bias" may be technically accurate, but it is not actually useful. And it's more a reflection of the writing profession as a whole than it is of this particular place.