If you started awarding assists for passes leading to fouls, it shouldn't matter what the outcome is of the FTs. It was still your pass that led to the positive outcome of a foul being drawn.
Missing a basket isn't a positive outcome. Getting to the free throw line, most of the time, is because someone fouled you to prevent what would have been an easy make. At the very least you pushed closer to the penalty or pushed someone closer to fouling out. Regardless, it's still a positive outcome.