Being frequently the instigator of such topics, often standing nearly alone on a side of some of them, and witnessing the mechanics of the door, I would be remiss if I didn't speak my mind here...
I feel obligated to apply my standard litmus test when it comes to evaluating a nay decision:
What's the actual harm (in letting a discussion continue)?
Are more donations needed for storage space or memory? Is the server overloaded? Was the value of the thread negatively impacted? Why do only early arrivers to the party get their say? Sometimes new arrivals want to voice their position on the matter before expanding as a means of identification, perhaps?
IMO controversial topics are often the most worth discussing because their resolutions usually have a profound impact (relative to non-controversial topics). How else are they ever going to reach fruition? If the congresses and courtrooms of the world avoided controversial topics, where would we be? Appropriate venue is, of course, important, but I don't believe that applies in many of the closings.
Let me give you an example of where I've seen the door slammed:
IMO this topic had nowhere near "run its course", so much so in fact that it was running parallel to a topic that I created for the express purpose of a resolution:
One might argue there's redundancy that could be avoided (though the two features being suggested were different while addressing the same issue).
I know, "heavy is the head that wears the crown". So I'll close with a scientific statement of something I believe:
The height of aspiration of a community's influence is positively correlated with its tolerance of controversy. This is a peak with the other side being the negative impact of personalization of standpoint.
I can say firsthand that you guys/gals at Let's Encrypt tend to be pretty fantastic when it comes to tolerance of what could understandably be considered criticism from many a direction. It's one of the bedrocks of this community and what I believe keeps a great organization on its toes, so long as there's an eye for improvement.