I cannot answer on behalf of the WB collective as I am and we are not a hive mind. I will give instead my thoughts, impressions and opinions.
Is WB SE killing stories? Answer: Only if you let it.
Too much worldbuilding, too little everything = Bad story.
Agreed absolutely. The exemplar of this is Orion's Arm Universe Project. There the worldbuilding has reached the stage of choking out what could be interesting story scenarios by rigidly adhering to the fidelity and the axioms of its conceptual universe. The Project has been around long enough that its worldview remains stuck in the nineteen-nineties.
On the other hand, the real value of the Project is that it sets out, in detail, a considerable number of scientific and technological concepts that can be used by the canny science-fiction writer. You want faster-than-light travel via a wormhole network for your story? Well that's explained comprehensively, so borrow all you need and discard what you don't. In that sense, it is an excellent resource and should be used as such. Try setting a story there and it's too much hard work for little reward.
Now WB SE is itself an excellent resource for ideas about specific elements in building worlds and the frameworks for imaginary worlds. In essence it functions more as a test bed for assisting worldbuilders to determine whether elements in their worlds are workable or not. What it is not is a resource for developing stories.
Now in doing so, this process isn't perfect, the agents doing the sifting, sorting, sieving and selecting in terms of the criteria to determine whether a given question is suitable for WB SE, are fallible human beings. Too often I suspect many questions are closed not because they fail to meet WB's criteria but because the VTCers fail to understand the criteria themselves either because they have never read the criteria or if they have haven't understood them at all well.
The reasons I say that is that in one of my careers I was a decision-maker who had to determine matters governed by Acts of Parliament and the outcomes had to be based on legislative criteria. This is especially so when the decisions I made could be reviewed and overturned by internal review conducted by my department, review by the Office of the Ombudsman, and in court. Indeed I had to defend my decisions in court on only one occasion. This means I hold much more considered standards for making decisions. All the criteria have to be properly met and often the benefit of doubt has to be exercised.
However, the question & answer methodology here at WB SE isn't designed to build stories, even though there will inevitably be worldbuilding elements proposed in questions that do go towards constructing a story. For example, asking about "How to overthrow the emperor of the universe when she is protected by an impenetrable force-field?". This is likely to be closed as story-based. Whereas if the same question had been posed as "How to penetrate an impenetrable force-field?" then the question would treated as being about a worldbuilding element, in this case, the penetration of an impenetrable force-field.
What I am trying to suggest is that questions need to be framed so they can be asked in a way that fits WB's criteria for questions and answers. While a querent can do all they can about their questions to try and ensure they fit WB SE's criteria, they have no control over the criteria that may be applied to it in terms of whether it meets those criteria. This does mean quite often some WBers seem to be looking for criteria to close when they could have spent more time putting effort into improving questions.
Is this an easy process? Absolutely not!! Having tried to suggest ways of improving questions, it is too easy to fall into the habit of dismissing questions with a snarky comment. I have no doubt I have been as guilty of doing this as anybody else here. Thinking about how to improve questions is hard work. Much harder all too often than answering the question itself.
Does WB: SE hinder the creation of good stories, and slows down the
workout of a concept, with its ... Q&A style?
Quotation above edited to remove unnecessarily emotive phrase.
My answer is only if you let it. Starting with the proposition that WB SE isn't in the business of fiction construction, it's possible to ask what is its relationship between storytelling and worldbuilding? Before I answer my question, we need to realize every story exists within its own fictional universe. Some facts and factors of the real world are excluded or omitted. For example, people usually don't die of cancer in a comedy, but they do in nitty-gritty realist fiction. On the other hand, they contain things that don't happen in reality: private detectives solve murders or people have super-powers.
Every story creates its own fictional world in which the events of the story can unfold according to the internal logic of both the story and the world within its events exist.
The trap some storytellers can fall into is in focusing too much on the worldbuilding, to the extent that overzealous adherence to a concept blocks the progression of the story.
How this can be circumvented I will illustrate by the following.
Kevin Grazier who was scientific adviser to Battlestar Galactica said he expected scientists who had watched the show to come up and admonish him for getting the science wrong. Instead what he found was they usually came up and said to him something along these lines: "Ah! I see what you did with this scientific concept. You assumed x, considered y, and ignored z." Smiling and nodding in agreement with what he had done.
Greg Egan said that when he was going to introduce a piece of nanotechnology into his fiction he could either spend the whole day calculating the thermodynamics to see if the concept worked or simply assume it worked and get on with the story. I chose the latter.1
Having noticed that Joe Haldeman creates most of the science-fictional devices in his fiction by postulating the rules about how they function I called this approach "Haldeman rules". For example, in his 1975 novel Mindbridge where the rules for the Faster-than-light teleportation technology are explicitly stated. When I mentioned this concept of a rules based system to Sean Williams, I remarked that he uses the same technique in his science-fiction too.
If a writer develops a set of Haldeman rules to cover their version of FTL travel, time machines, antigravity or thermonuclear cheese, then provided they stick to the rules this will engender internal logical consistency to make their story more plausible. It also makes the writer's task of worldbuilding easier.
Was WB: SE a good idea, or it was a mistake?
Strictly speaking, the tenses in that quotation should be present tense, but that's not important.
The answer is yes and no. For what it intends to do, WB SE may not succeed as well as it could, but what it actually achieves makes it invaluable. Its main failure is in fulfilling its own set of criteria comprehensively and most of the time. Where it does succeed is in creating an extensive resource of ideas, concepts and putative frameworks for worldbuilding. Many of its answers are exceptionally brilliant providing amazing insight into how worlds can be built, often overthrowing conventional misunderstandings, and helping to raise the standard of what goes into worldbuilding. Does it always succeed in doing so? Absolutely not, it is a fallible human institution and fallible human institutions often fail, but frankly they are all we have got.
If you expect WB to help you build a story, that's not what it's for. But if you use it as a way to guide you through the darkness, then it can be invaluable.
1 Greg Egan, personal communication.