Posts can be deleted until improvements are applied, including being deleted by the post author. This seems to be an anticipated usage, from the fact that a user with the Curate ability can see deleted posts. Suggested edits can be part of the recovery process of a deleted post.

Deletion isn't for putting posts aside until they are improved. That's what closing a question is for. Even then, authors rarely make the requested improvements, and closed questions are rarely re-opened. Deletion is for post that simply don't belong, like spam, vulgarity, or otherwise outright not fitting as apposed to being only poorly written. At some point we have to declare something irreparably broken and move on. That's what deletion is.

Declaring something irreparable and moving on is one use for deletion. In some communities it may be the only reason, but some communities have other reasons. For example, on Code Golf an answer that is found to be incorrect may be deleted by the author until it can be fixed.

Regardless of the expectations of any particular community, the software is designed to support editing deleted posts. If either of you would like the software to be changed to prevent the editing of deleted posts that seems a different feature request, so I'd recommend raising it separately.