The question issue that people ignore when they tell you "nipple shields will decrease your milk supply" is "compared to what?"
Most women start using nipple shields because they are having significant problems wither with pain or with a baby who has difficulty in attaching. Milk supply works the same way with shields or without shields - the more you take out of the breast the more you will make. If your baby is not able to attach properly and feed effectively without a shield that will have an impact on your supply. If you have to express and feed with a bottle because the pain is too much that will affect your supply. The shield is only the temporary solution - you need to look at the problem and seek help to fix it. (and there is help out there) Sometimes it is only time and growth and development that will fix the problem. More often there are some simple strategies that will help. Nearly every baby comes off the shield relatively easily when the problems resolve.