G'day Keith,
Alternatively, we shouldn't shut down a system that works reasonably well and performs an important task without having something that works better.
Ay, there's the rub. I get the feeling those arguing for shutting down AfD don't feel that it *does* perform an important task. Rather, they'd prefer not to have deletions at all (other than CSDs, which already apparently go too far ...).
If you've noted an element of sarcasm or, at the very least, tongue-in-cheekery from David, Anthony, etc. then this may provide a plausible explanation as to why.