Even then, there are a lot of "best answers" from six years ago that are really poor by today's standards. I think it's good to keep them open so people can improve them if they like -- or fix them if they're incorrect, which has happened before. I think there would be so much exception that would have to be made for this to be helpful, that it ceases to be helpful at all.
Worth mentioning there's already a function on the site that requires new answers on old questions to go through the admin if you are under a few hundred points, which catches a lot of the pointless stuff. Maybe that could be extended for comments.