They become irrelevant. In trying to become so much like the world to win the world, the world influences the soft-sellers more than they change the world. They wind up just as useless and the strident salt-churches. But there is a bigger problem with the salt-church motif. Though it improves on the light-alone idea, it is simply not aggressive enough.
Salt stops evil, but falls short of providing a model for transforming evil into good.
It stays decay in the meat, but is unable to make the meat into salt. Stemming the tide of moral decay is a worthy goal, but it falls short of the total redemption God has in mind. God intends to redeem all of creation, not just stay its decline. Salt alone is still not adequate to paint the full picture of our relationship with the world. Perhaps there is a third model which fills out the picture.