This comment for example, after about a week or two most of the visibility and interaction of it will drop to zero. At that point, this comment should expire and no longer exist.

That's an incredible naive and egoistic take. Think about all the knowledge that is getting lost by applying this approach. How many times have you searched for some obscure thing and found the answer only on some five years old reddit post? That information would be lost for ever if you had your way.