The Security.SE Help Center specifically says:
You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page.
The idea of an AI which is so powerful to automatically solve CAPTCHAs and to automatically write useful comments based on a given context is not currently practical. While there are questions in this SE about the impact of quantum computers on cryptography, they are more practical because the technology is partially available and because basic ideas are already present (post-quantum algorithms).
Your question, on the other hand, takes for granted something that we cannot really understand, and that would bring so many complications that a definitive answer cannot exist (i.e. not answerable).
An AI with that level of sophistication could pass the Turing test, which means that it would be indistinguishable from human users. This, in turn, would mean that the spambot problem would be equivalent to the trolling problem on online communities, if the AI comments were not random but still not constructive, or to an even more complex problem, if the comments were also constructive.
For example, what would happen if human users rated as "not constructive" a comment which is actually meaningful, but conveys an opinion contrary to that of the majority of the users in the community? This is possible, and it already happens in some online communities. How could you distinguish between a constructive comment downvoted for this reason, and a comment downvoted because it is generated by an AI spambot?
Would it even make sense to talk about spam, and if so, how could we draw the line between this kind of "spammer" and a user with a different opinion from the majority?
This problem would not belong to information security anymore, not even remotely. Since you asked about an AI writing constructive comments, this is the only possible problem that would arise, and because it would not be an information security issue, your question is off-topic.