`git reset HEAD~` doesn't feel like that much of a contortion to me. It's the destructive change that requires more contortion (`--hard`) which feels fair. Maybe this is stockholm syndrome though.
The way I think of it, there's basically three copies of the file in play: in HEAD, in staging area, and on disk. I cannot trust my memory to remember which variant of "git reset" copies the file in HEAD to the staging area, which variant copies staging area to disk, and which variant copies HEAD to disk (in all cases, the third copy remains uninvolved). Getting it wrong potentially creates unrecoverable data loss. And, unfortunately, this is one of those cases where reading git's documentation is less than helpful.
Combine this with the case where "I want to break one commit into two commits," where now I have to worry about making sure I know if the command is going to change the revision HEAD points to. At least there, the old commit will still exist as backup in the invariable scenario I screw something up.
You can just use the tip as your staging. Use interactive amending to move changes from the working copy to the commit, and when you want to "commit", finish up the message.
hg actually has an "unamend" command (part of the "uncommit" standard extension) which... reverts the last amend. Rather than having to remember how to contort reset into the right shape to move changes back out of staging without destroying everything.