There are some interesting angles on this.
For content publishers/writers who are (more or less) publishing their work on the blockchain, indefinite editing means I could publish a short story on steemit, gather what rewards/accolades/spambot messages I may, then come back in a month when the story is long forgotten and....erase it. Or just leave a tiny teaser paragraph. Or a link to something else entirely. It does take away a certain feeling of permanence to the blockchain. Revisionist history.
However, it would also give me full control of my content and full ability to 'republish' that story with another (real/mainstream) publisher since the original would be long gone and Google would no longer see it (although the original versions are still stored on the blockchain, of course, they're effectively invisible for all intents).
So, it could be used for both good and evil.
Yes, yes they are. That is the key which, for me, overcomes all objections to editability.
Writers live and die by their reputations. If you use this ability for evil, your reputation will suffer and your impact for evil will diminish. Watchdogs will be able to unequivocally display your perfidy.
Of course, some of the scenarios you've described are open to moral interpretation.
I would maintain that this could be an entirely legitimate option, redirecting the "hoi polloi" to Amazon or wherever you may care to transact, exchanging your valuable wares for Steem or SBD.
In other words...
This is the best frickin' thing to happen on Steemit since,
like, for-EV-er!
It's all good!
Thank you for an outstanding comment! :D