Humans change things when enough humans want them to change. If there are not enough in favour, the change fails (perhaps a revolution fails too). Sometimes thing change as they are manifestly simple and obvious.
Moving to a unified method of counting and measuring is in the second category. Trade benefits. Everyone benefits.
For better or worse, the world has agreed to travel being based on a 'seat from A to B'. How that is priced varies enormously but, since 1945 (or so) - weight of pax has not been part of the price calculation. On any occaision that this topic is raised in the USA - more pax complain than support it. If you can find a way to change the USA way of pricing airline seats to the benefit of the airlines then you are sitting on a gold mine. If it only benefits the pax - forget it.