> So ideally that question should have answers that apply to the new question as well.
The point is who decides. If you ask a question and I flag it as a dupe, I might think the answers on the other question apply to yours, but only you know whether they solved your problem or not.
> I've no idea if the rate of bad duplicates is so much higher than I observed,
Sure, and neither does SO! They didn't even measure it. They only looked at the signal "does somebody with points think these questions are similar", and discarded the signal of whether the new user got any value out of the site, and I think that's what did them in.
> The point is who decides. If you ask a question and I flag it as a dupe, I might think the answers on the other question apply to yours, but only you know whether they solved your problem or not.
The site is not about solving your problem. The site is about answering your question.
By policy, whether a question is a duplicate depends on whether answers at the target answer the question that is actually present in the "question" post. The details required to adapt that Q&A to the originally motivating situation are irrelevant.
In fact, there doesn't need to be an originally motivating situation in order to ask a valid question.
In fact, you don't actually need to know an answer in order to ask a valid question. It is perfectly acceptable, and even encouraged, to ask a question you know how to answer already, and provide the answer. (But both question and answer must meet standards, which are independent of who is asking and who is answering.)
> Sure, and neither does SO! They didn't even measure it.
This fundamentally can't be measured. Especially not when the claim of "bad duplicate" usually comes from people who are demonstrably not interested in what the policy is.