Who gets to decide which comments get to stay and which comments go?
There are no reliable mechanisms for the community to rank comments. They can’t be downvoted. They can only be flagged to be removed, not to be “pinned”. It is not feasible to ask a few volunteer moderators to singlehandedly curate hundreds of thousands of comments and preserve the “valuable” ones with no real consensus from the community, especially when the tools available to them for moderating comments are not designed for that task.
If the community finds comments that are too valuable to risk losing, the only sanctioned way to handle that is to capture that information in an answer or question on the main site or meta.