It’s not difficult to implement, and we want to get to it, but
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Not that many complaints about this. “Would be nice to have” sure. So would lots of stuff!
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I worry about the UI. Where are you going to set the “wiki” flag on the post? Mock me up some UI. Where would that go? How would you “know” that something is editable other than seeing the edit pencil (inexplicably?) on a post you didn’t write? Wouldn’t that make you go “huh?” and possibly worry that something was wrong? It’s also kind of a fairly advanced feature we don’t want newbies accidentally getting into.
It’s not a common feature in a forum, there is little to no expectation that others will or can edit your posts. Doesn’t mean it’s a bad feature, just rare and narrow.