You are a moderator I assume so please don’t think I am disagreeing.
I very much understand what you are saying about changing what the original post says. I agree that is not good. However it would not be a problem if the post was added to and that type of thing is often done in other forums.
I agree that we should read the entire thread. However in this case the original problem did not exist in the original post. The original post was no help. Something should have been done about that. In other forums moderators do edit posts when appropriate. In StackOverflow, the most popular forums, members do ask that a question be revised when it is not clear.
If you don’t want the original post to be edited then the forum software should not allow it. That is impractical so the question becomes what is appropriate and what is not.
While I agree that there may be odd occasions when it is desirable to edit an opening post for clarity, we have found over the years that making such edits generally leads more to confusion, which is why, as a general rule, we ask members not to edit posts once they have been replied to.
I understand that other forums have different policies. Ours are built on experience of what works here. It doesn’t make them better or worse than those of any other forum - just different. StackOverflow is not so much a discussion forum as a question-and-answer site, so it is not surprising that their approach differs from ours.
The issue was identified through the ensuing discussion, which is typical of threads here. As mentioned, this is not a question-and-answer service, but a discussion forum. We therefore expect those interested in participating to read the whole discussion before replying, to ensure they have the full picture, and also to ensure they are not repeating advice/information already given. This approach generally works well.
If there is a problem with a post then any member can flag it for moderator attention - for example if code needs formatting to be visible. However, we are all volunteers and our time is limited, so we expect members to use this facility judiciously.
Yes, StackOverflow has the disadvantage of intentionally limiting and explicitly discouraging discussion.
Yes, if we can help then we should first read the entire discussion. (Occasionally discussions grow so much that is impractical to read all of it and perhaps I should say something about it.) However the problem is that if we cannot help then it is a waste of our time to read the whole thing to determine we cannot help.