If that were a hard-and-fast rule, you could get situations with a plane full of schoolkids, elderly, famlies, etc., that could not take off. Are you really going to force a family with a child, senior citizen, or disabled person, to split up so that one adult can man an exit door that is extremely unlikely to be used on either takeoff or landing?

Many if not most emergency landings come with enough warning for FAs to shuffle people around anyway.

And I'm not at all convinced that an "un-briefed" non-exit-row passenger who makes it to the door first in an evacuation will do any worse job than a "briefed" exit-row pax.

I'm all about extra leg room and spend a lot of time in exit-row seats, but I'm against making FAA rules for the sake of making rules. The unintended consequences are often worse than the supposed benefits.