You have a point there. To defend your copyright on your own original works, your work's copyright has to be registered. If you didn't register it, you can't file a DMCA claim. But, if someone files a DMCA claim against you, and they either don't have a registered copyright, or their copyright is registered later than the time stamp on your published work, they would probably lose the case against you.

One trick that authors use is to put their manuscript into an envelope and send it through the mail to themselves. When it arrives, they don't open it. They file it as proof of date authored. That's a wise thing to do even before publishing or sending to a publishing/printing agent. I suppose an email to yourself would serve the same purpose, but the text/files in an email are public the second you send them over the internet. Email is cheaper and the risk of anyone nabbing your work is low. I still would prefer a printed hardcopy on file. Absolutely no one reads your work and it doesn't likely leave your local post office before returning to you. Another option is to have a removable drive for storing the works you want copyright protection on. There will be a timestamp on the files and if you need to take the files to court, you take the whole drive, so the timestamps do not change, as would be the case will a copy of them. You could store them on your computer and take a screenshot showing timestamp as well. For defense purposes, any of these techniques would likely be recognized in a court of law as proof of original ownership. But as you point out, to file a DMCA claim, you work must be registered.

On Steemit, because an author only has a one-week window to collect gains from a work, it's not a good platform for serious authors, working for a living. Authors cannot edit (delete text) or delete posts after 7 days. Essentially, after 7 days, Steemit effectively owns and controls your work. According to the Steemit Terms of Service, "You retain ownership of and responsibility for Content you create or own ("Your Content")", so, if you - the author - choose to delete the content, you are entitled to, but not after that payout window closes. That's contradictory. Serious authors on this platform may consider editing the text of their own posts (delete content) just before the payout window closes to manage their copyrights and that includes the rights to republish at a later date. Having an existing copy on Steemit would provoke the plagiarism bots, if you reposted the same poem or short story at a later date, for example.