Content posted to the web is usually openly accessible to all (unless protected by a password, paywall, or similar restriction). But that does not mean it is freely copyable by all. Such content is protected by copyright in just the same way as if it had been published in a book of essays by various contributors.
Unless the copyright holders (who are likely to be the original authors, but might not be) give permission, or an exception to copyright applies, copying such content would be clear and obvious copyright infringement, and any copyright holder could sue for damages.
Permission could be given by publishing the content under a permissive license, such as a CC-BY-SA license, or any of many other available permissive licenses. Or a would-be reuser could find the copyright holders and ask for permission. If the holder cannot be found or identified, or does not respond, then no permission has been granted.
In the US the main exception to copyright is fair use. See this answer and other threads with the fair-use tag here for more on fair use. Since the question seems to contemplate using the whole of the posted content, since it might well damage any potential market for that content, and since the use does not seem to be "transformative", nor used for criticism or comment, a finding of fair use for this situation seems unlikely. But Fair use findings are very much fact-driven, and the exact facts do matter. Thus I cannot be at all sure whether a court would find this toi be fair use or not.
In other countries there are a variety of exceptions to copyright, and I have not come close to reviewing them all. But none of the ones I know of seem to apply to the situation described in the question. Many are narrower than the US concept of fair use.
I fear that without permission, copying this content would be infringement. However, it would not be infringement to create a site that includes a link to the existing content, and a summary or description of that content, along with new content, including comments on the old, with brief quotes to indicate what is being commented on.
