Or a bot that browses discussions and if any recent ones have > 100 comments automatically sends the link to archive.org to be archived and then add a comment with the link of the archived copy
Bots are heavily discouraged here. (Almost totally forbidden.)
Perhaps if the bot sends the page to the archive immediately but instead of posting the comment immediately waits a few days (until the last day comments are allowed), it will not so controversial because it will add no noise to the conversation.
It's better to send an email to the mods with the idea before implementing it, because the bot may get banned.