The 7-day editing rule is the bane of my existence. Seriously, though, not being able to edit your post after 7 days is nothing trivial and its not just about correcting grammar/writing mistakes. It exposes both the author, and possibly Steemit.com, to potential copyright issues.
I mean, we all post pictures in our blogs. And I'd bet that a lot of us, especially in the beginning, might not be 100% certain of our rights/license to post pictures, or that these pictures were attributed properly. And that's no small thing. Who's to say that a photographer somewhere will not take action in the future?
The ability to edit past 7 days just means that we can reduce risks. This way, if you find out months down the line that a picture you thought was license-free is not, you can proactively remove it from your post and save yourself the potential legal problem.