I think @dfabulich touches on a critical distinction: there are (at least) two different kinds of edits, which the Discourse software could potentially treat differently in the future:
- Minor corrections—e.g. typos and spelling.
- Content correction and clarification—i.e. “setting the record straight.”
For (1), the e.g. 5-minute editing grace period makes sense. I personally make heavy use of this feature, and agree heartily with @codinghorror about the “printed in bright lights” effect.
However, I think using the editing grace period to address edits of type (2) is not the best solution. Instead, imagine if Discourse had a sort of “track changes” feature where edits beyond the grace period did not erase anything, but instead struck things out and highlighted new things with e.g. yellow-background CSS. This would be amazingly useful for addressing misinformation and clarifying previously unclear statements, in a way that did not invalidate later posts offering that feedback in the first place. It would also assuage the fears of those with “edit feature hesitancy” who (at least in my community) have pushed back against the fact that posts can be edited at all, for exactly the reason that erasing past statements can create more confusion than it solves.
As for why not “do that in a normal reply, like a normal person”: in my experience, normal people are reactionary. They read something at the top of the thread, and don’t always notice corrections in later posts—especially in dense/busy topics. It would be much, much better to correct the OP itself, nipping problems in the bud so to speak.