I think some tweaks would be very helpful:
1- Don't keep multiple drafts for a subject (post/pm) just keep the latest version.
There's a reason it's called a draft (preliminary stage of a written) sometimes you may want to recover your previous writing and with the draft that can be made possible. I once wrote a topic them preview it later I decided to edit my post before posting to make it more informative. I ended up deleting my last two paragraphs and when I preview again the sentences I added, sounded so dumb to me and I needed my old work back. The draft because very useful and I got my previous work back so I don't support this suggestion of only keeping the latest preview.
For a draft to get filled up you need to be creating an average of 14.3 previews daily for a week so if that's the case then you need to slow down and get some rest. No offends OP
Not taken.
14.3 is nothing, I do a lot of edits and 7 days is nonsense.
For a serious writer it is completely possible to suspend writing a post for a while for may reasons, doing more research, finding some lost reference material, carrying out some calculations, re-thinking some points, ... it is more important use-case compared to simple redo. A compromise would be keeping a maximum of 3 last versions and removing the 7 days window for new posts/topics that are not committed yet.
I copy long posts to a file once in a while when long posts take a lot of time.
It's also useful if I ever mess it up editing later.
My Firefox usually restores open editing too when I restart it.
Have you checked your browser cache for a copy of your work?
You are right many browsers support restore feature but it doesn't work properly always and without it you get nothing.