An oft-missed benefit of being a partner is the industrial work share that comes with it. This reduces the cost of the Program to countries. For instance, the U.K. Will get around £3 back for every £1 put in, provided the total numbers forecast are eventually ordered - clearly that's not guaranteed but there's a way to go yet.
If you no longer want to be a partner, why should you retain the work share? Far better to split that amongst the committed nations and concomitantly reduce the cost of the Program to them.