In our home, the five second rule has always just been a joke. Someone will say it and we'll laugh, but never follow through. I have two very healthy grown sons (18 and 20) and we never intentionally let them eat something that had fallen on the floor. They attempted to eat their share of things as toddlers, (dog and cat food out of friend's pets bowls for example), but I would never pick up something they dropped and give it back to them. It's just not worth the risk. Kids are going to eat their share of dirt, sand, bugs etc on their own. We don't need to enable them.
To me, we're overthinking this. Common sense says, the ground is dirty. Yes, so are door knobs, bathrooms, money, remotes, etc, but I dont' eat off of those either.