Final Death of Necroposting

I don't understand why you can't just have it (assuming this is possible) warn people when they post within a thread that's over 21 days old, instead of closing it automatically. That way all the exceptions don't need to be dealt with personally, since the person posting is posting for a reason, and then any newcomer who posts in an old thread by accident also knows better.

This was the approach I tried first, it didn't play well. A passive warning is useless, it'll be skimmed right over and ignored, then be replied to with "but i didn't see it" when infracted. The checkbox to submit a reply on older stuff was much preferred but didn't work right.

Who said the posts would be spam? They can just make contributing posts just for the sake of keeping the thread from getting locked rather than sheerly for discussion, and it can't be considered spam at all because the posts have some level of meaning pertaining to the thread.

In the scenario you've described there's no problem, so what was the point of bringing it up? If they're contributing and it keeps the post alive it sounds win-win to me.

Is it really that much of a hassle for you to sit down and think about which forums are needed to be exempt from this restriction?

I could've, but you'd just complain I was being dictatorial anyhow by not including the community in the decision making process. You're included now, enjoy.