In 1981, after reviewing a series of accidents, the FAA determined that an aircraft below 10,000 feet is in a critical phase of flight and imposed the sterile cockpit rule. When the cockpit is sterile, no member of the aircrew, including flight attendants, are allowed to engage in any activity that could, distract any flight crewmember from the performance of his or her duties or which could interfere in any way with the proper conduct of those duties.
The takeoff speed of the latest 747 is 160-180 miles per hour (257-290 km/h). What do you suppose would happen if the plane suddenly came to a halt, or twisted off of the runway? Just like the groceries in the back seat of your car come crashing into the back of the front seats, everything is thrown forward.
If the tray table in front of you was open, you would smash into it, posssibly bisecting your chest. That laptop or tablet you are holding would go flying forward, or if it was in the seat pocket in front of you, your knees might go into it, shattering the glass screen and embedding it into your flesh. And if you do need to quickly evacuate, its much easier if you dont need to worry about where youre going to stow your gear before you can get your butt out of the exit to safety.
What about wearing noise-cancelling headphones? In a sudden deceleration they also could become a projectile, hurling over the seat in front of you. Not to mention that if you have them on you wouldnt be able to hear the brace or evacuation instructions endangering you and your fellow passengers. If your seat was reclined, it could some slamming forward, ejecting you or it could block the person behind you from getting out of their seat.
Yup, the original rules were based on solid science. The new rules are based on passengers whining about not being entertained for 10 minutes.