Originally Posted by LarryJ

Perhaps for new airplanes but that would make most of today's narrowbody airplanes financially unviable.

The B737, the most prolific jet airliner in the world, has seats with a width of 17.3". If you were to bring those up to 18" you'd lose 4.2" which would either make the aisle so narrow everyone would have to go down it sideways or you'd have to remove 1/6th of the seats on the airplane.