Okay so firstly; counting headshots is fine, but determining reaction time is a lot more complex. It would need a way to know, and be sure, exactly when an enemy becomes visible clientside. This could be one pixel spotted through a crack in geometry where something like tracing to every hitbox corner would fail (and be quite expensive to do for all players for every frame, re-simulating and emulating 500fps serverside). And spamming or predictive shots would read as 0 reaction time, but you can't dismiss these as this is what cheaters are doing. People's reaction time (when measured correctly, on gamer level specs) could reasonably be 120ms but that's a minor tweak. It would also need to do full accurate rewind for your ping to resim exactly what you saw with a GPU render of every frame (so each server has to basically have the power of 10 high end gaming PCs). You'd need something extremely robust for this which may or may not be possible or viable and very server intensive. I'll grant that it's plausible to do though given unlimited resources, and you mentioned it as an average which would *mostly* avoid any legit players. There may be some exception cases but those could perhaps be manually reviewed.
So now that you've given one specific, which is practically impossible but theoretically viable:
As you say, the cheater makes it start shooting the body when a bodyshot will kill, or they add some arbitrary delay. You still die 100% of the time to them, never able to get a kill until you drop that threshold down. Now you've increased the number of false positives and added more manual reviews needed.
So after this first implementation there are just as many cheaters, with the potential of a handful of legit players banned. That is a failure in my eyes. You can repeat this process and the answer remains the same; more legit bans, cheaters adapt and continue.
If you keep doing this, eventually you will start to stunt the viability of cheating/boosting/etc, but you have also started to kill the playerbase because legit players are getting banned regularly now and don't trust the AC. Way way way before any preventing any cheats (but maybe making it marginally less appealing).