I had posted a question, but later realised through the comments and the answers, that my question's fundamental assumptions were wrong.
When this happens on other platforms, I delete my question but I tell people to not delete their answers. But I don't think that is possible in Stack
So should I delete my question because it isn't a proper one, or should I leave it so that other people with similar misconceptions can see the answers (and so that the people who answered don't lose reputation)
No,
Do not delete questions just because you got an answer.
Ask yourself: Is it likely that a future visitor will follow a similar thought process, make similar assumptions and end up with the same wrong conclusion as I did?
If you expect that might be true then your question (and its answers) have value and some day in the future someone else will benefit from your and others effort. Check your question to make sure it has the key elements clearly worded so Google can index it.
On the other hand, if you think no-one will ever land on the same issue again then a question and the answers don't have lasting value except for you. Deletion is then an option, assuming you didn't receive positive scoring answers.
It is worth noting that deleting questions can cause the quality mechanisms to kick-in. Asking and then deleting too many questions risks your account getting either rate limited or quality-banned, allowing you only one question every six months.
Optimize for future visitors is the best advice I can give.
If the people who answered your question have reputation to lose from their answers then you are unlikely to be able to delete it.
In any event, I think you need to be mindful of existing answers whenever you edit a question after they have been posted. It is fine to improve your question, as long as you do not invalidate any of its answers that address what you had asked.
However, as commented by @rene, a better place for addressing any incorrect assumptions in your question, that influenced their answer, may be in a self-answer to it that starts by pointing out the incorrect assumption(s).
In most cases, I think that the proper option would be not to delete the question and to edit it, adding a sentence or two in which you explain that now you think that the question's fundamental assumptions were wrong and why.
