<p>In the answer of user @scortchi we read</p>

<blockquote>
  <p>Down-votes do tend to be sticky, which is a pity when the down-voted
  poster makes the effort to improve their post. I don't see any reason
  to suppose this is due to anything more than people's disinclination
  to keep on returning to a post they've down-voted to see whether it's
  been improved.</p>
</blockquote>

<p>This is exactly how we treat a crime: we punish the wrong-doer for the crime (bad post) and we also hope that punishment will contribute to better <em>future</em> behavior (future posts). In the factual world this may be realistic since in most cases an action that has exceeded the threshold for a crime cannot be altered/undone. <em>But SE questions can</em> - and by not following up on our downvotes, rewarding by removal of the downvote those posts that were improved, we forego the opportunity to beneficially exploit this additional flexibility.  </p>

<p>When individuals (users) use rights &amp; granted powers (downoting) in such an incomplete way, and there is no prospect for a change-of-mentality on the horizon, usually the only solution in sight is to have community representatives take up the duty to clean up (this is a large part of what public sectors are tasked with doing across the globe). In our case, our public sector is the moderators, and it would mean moderators being able and tasked with monitoring downvoted posts and removing downvotes from those that were improved.</p>
