Stoop, you may be right with your comments, but all airlines work on the basis of 50 kgs of weight per passenger including baggage as they need to work on these figures to know how much fuel is needed to fly to its destination,and also book the fuel tanker long before you have checked-in for the flight, as an example a boeing 707 would need 20 tonne of fuel without passengers or baggage and or cargo just to get off the ground, most short haul aircraft have enough fuel on board to get back to its base without refueling at its destination point, as some airports will charge them an arm and a leg to refuel, and back home they buy fuel on the spot market many months ahead of today's prices, also aircraft fuel is not the same as you have in your car......its kerosene and the price per liter is much less than car fuel.
The reason why airlines do not want you take hold baggage is two fold A/ it takes to much time loading and unloading and it effects the turn around time,also they then have to pay ground handling charges as all the time the aircraft is on the ground its not making money, and B/ the less weight means less fuel, but at the same time they have charged the passenger the full rate as if he/she has a weight of 50 kgs, all the extras ie: speedy boarding, excess weight, sports equipment, food and drinks, is where they make their money, as the crew earn a bonus on the extras including overweight baggage, also some airlines will charge on board food and drink prices in euro then give you a very poor exchange rate when you pay in sterling.
Air miles are inflationary proof 300 air miles from london to paris will always be 300 miles but should they do what is being suggested it will be a very good money earner as 1 pence per kilo per mile today will result in being 5 pence tomorrow and then more when they want to earn more revenue, us the unsuspecting consumer will just put up with it, and will you know that's its gone up 2 p or 3p per kilo per mile, it happens already all the time in any supermarket in the uk, do you complain no, do you carry on buying the product yes!!