The problem is that the top-level post is still there, and Facebook will often choose to show it to people without showing them the reply. Or people will read the big text and skim over the reply.

At the other extreme, full deletion has different downsides:

If you're deleting something with replies, those comments disappear, even ones people may have put a lot of work into. Plus people are even less likely to know you retracted your claim.

For simple retractions, where once people know you don't endorse it anymore it's no longer harmful, an edit can work well:

It's not confusing, and people can still read the comments if the discussion is helpful.

If you really don't want people to gloss over your retraction, you can edit to remove it fully:

If people really want to see the context, they can look at the edit history:

In extreme cases, like if you accidentally outed someone, changing the privacy setting on your post to "only me" lets you keep anyone else from seeing it:

This way if someone has a comment they wrote that they'd like to still have, they can write to you and ask for a copy.

If you have a comment in a nested thread where you need to clear the edit history, delete your comment and post a new one like "I deleted my comment above because...".

If you have a top-level comment you need to delete, there aren't any great options. The best you can do is manually screenshot the thread in case someone asks for their comments, and then delete it.

So, please do retract things when needed! Just be thoughtful about it.