Being 60kg and rarely travelling with a checked suitcase I would love paying by weight.
But lets face it, it isn't going to happen. Too complicated to price in advance, recalculate if you gain/lose weight and/or pack more/less than expected, etc.
At best we might see a price based on a maximum total weight (say 100kg) with surcharges being applied at the airport if you exceed this. Even this would be complex - Your 40kg kid could take three 20kg suitcases. (Remembering that volume is just as important as weight in the hold)
Nice idea but ...