This PSA can be a good idea, but only if done properly. Simply updating the original post isn't sufficient and may in fact make things far worse.
You aren't thinking of it from the standpoint of the outsider. Imagine you click on a thread and start reading it for the first time. The poster had updated the original post but didn't highlight what changes were made. Then you read 10 or so responses which make no sense to you because you have the updated info. Then you post that the first 10 people are idiots with their answers - and arguments ensue.
Of course, in reality, the first 10 people were not idiots. The true problem was that the original post was updated WITHOUT the update being marked.
So you need to update your PSA. People must:
1) Update the original post,
2) Mark the updated section, and
3) Not delete the old section.