There was never any restriction on posting answers unless you had explicitly and repeatedly violated policy. Posting questions and answers was a default privilege extended to everyone at account creation.
It did require a small amount of reputation (50 points) to comment on posts that were not your own.
But none of {questions, answers, comments} are "reply posts". That's just not at all how the site worked. It also comes across that you imagine that "helping others" on Stack Overflow entails writing something (either a comment or an answer) that addresses what the OP is asking. That, too, is not how the site worked. A Q&A site like Stack Overflow works by producing reference material in Q&A format.
The point is that other people can search it later, and get direct answers without having to parse through someone else's comment discussion, walking a beginner through debugging steps just to get the necessary information to understand the question, etc.