<p>A useful principle governing these situations is</p>

<blockquote>
  <p>Respect what the community contributes.</p>
</blockquote>

<p>This clearly implies that whenever someone has made a substantial reply to a question, the thread should remain visible to all. If the question or any answer has an upvote, preserving visibility becomes even more imperative.</p>

<p>As a moderator, whenever I have encountered a unilateral deletion of such a thread I have reversed that deletion and left a comment to the O.P. explaining why. I can't remember any situation where there was significant objection.</p>

<p>This principle applies, albeit more weakly, to threads in which there appear to be important contributions in the comments.</p>

<p>Other courses of action are possible.  For instance, one could unilaterally repost the deleted question and self-answer it.  Although I can't immediately come up with a strong objection to this maneuver, I don't like it for its artificiality.</p>

<hr>

<p>As with any important principle, there will be notable exceptions.  One that arises about once a year occurs when the O.P. discovers they have posted sensitive material.  In those cases we delete the question and also contact the SE team to remove the edit history.</p>
