I think the fact that this heavy duty disabling of a feature can be done via a plugin is easy enough. Is it more that just installing a plugin that makes this a problem?

Also since adding the option seems easy, try opening a ticket to see if a discussion there could reveal some potential issues with a default implementation, perhaps even propose a patch!

That setting will turn off comments for new posts. If you want to turn them off for existing posts, then there are other settings to do that. Basically, you can control comments to a fine detail and you don’t need any plugins or extra code to do that.