The “replying too much thing” is not meant to be a barrier to discussion, but rather subtle hints not to turn a civil debate into an uncivil argument. The whole software was designed with the psychology of the people using it in mind, to nudge people in a good direction instead of forcing them or relying on heavy handed moderation.
A large part of the last several years of Discourse’s development have been analyzing user behavior in other forums, finding what derails threads, what gets people heated, and all the other little behaviors that make a forum cancer and figuring out how best to guide people away from those behaviors. They know exactly what they are doing with these “suggestions”, they’ve carefully considered the balance between annoying users and guiding users, and in the case where user testing has shown they make a difference they err on the side of
teaching nagging users not to be inconsiderate assholes.
As a result, sometimes these little hints are extraneous, sometimes they help us remember to be respectful, sometimes they show you the thread that already exists that you should have used instead of trying to make a new one.