Since I hate being in the car more than just about anything on the planet I would say no distance is too short to fly.
Anecdotally I've flown ATL-JAX-ATL 4 times. The drive is between 5-6 hours. I've done the flight in as little as 39 minutes.
Just using your post as an example, Sir, but no one is accounting for the ground time connected to flying. A 4-5 hour drive, ATL-JAX is easily that much time door-to-door on an airliner. Your tolerance for wasted time to fly is admirable. I used to regularly travel from Hartford to Montreal, it was often quicker to drive, due to not spending 45 minutes to the airport, 1+15-1+30 checking in, security, then clearing in Montreal, collecting a bag, going to the rental car counter, driving to the office. All of which was another 45-1 hour. It’s TRAVEL time, not in vehicle time that matters.
I have flown ATL-JAX-ATL a few times and have also driven it probably 20 times. Flying:
Uber to train - 5 minutes
Train to ATL - ~20 minutes (drops off right in terminal).
Airport Time - 1 hour - (Pre-Check - this gets me to gate right around boarding at T-45) - lets say 1.5 hours without precheck.
Flying Time - 1 hour
Exit airport time - 30 mins.
So we are looking at 4-4.5 hours. The downtime in the airport and in the air gives opportunities to sit down, boot up and do some work. Yes there is possibility of mechanical issues or weather delays which could tank the whole trip. Driving
Google Maps (no traffic and no stops) - 4:56-4:58 minutes
Bathroom stops - 2 stops x 5 minutes = ~10 minutes assuming you are only using quick on/off rest areas which is not always possible.
Fueling at start and at arrival = ~10 minutes (I can make it to JAX on 1 tank but I will be ~20 miles from needing to fillip when I get there)
So we are looking at ~5:20 minutes best case scenario that does not even account for the possibility of hitting the horrific traffic south of Atlanta in Henry County that could easily add 30-45 minutes to the trip time, potential accidents, construction, speeding tickets, etc. Not to mention it is all lost productivity time where you are just sitting whittling the hours away staring at billboard hell and there is absolutely zero scenery or anything interesting to see the entire drive while having to maintain focus on piloting a 3000+ lb automobile for 5+ hours.
I would prefer to possibly save ~1 hour on the whole trip and arrive rested and relaxed. Every time I drive it I am exhausted from sitting in the car for so long both when I arrive in JAX and when I arrive back in ATL.