What will happen if I don’t' close the asd in my child's heart?

Shorter life span. Large and long-standing atrial septal defects can damage your heart and lungs. An adult who has had an atrial septal defect for decades may have a shortened life span from heart failure or high blood pressure in the lungs.
Roll the dice. The ASD removes the lungs as a screening mechanism for the stuff that may end up in the blood. Germs, clots etc that might otherwise end up tossed to the side can end up in the brain causing a stroke or infection. If you want to live with that, be my guest. Sorry if I'm too blunt, but some of this is just heartbreaking.
Rhythm problems. Rhythm problems with significantly impaired quality of life as an adult.
Depends. If the ASD is very small, then there may be little issues. But if large, then this can lead to increased blood flow to the lungs which can cause permanent damage. There is also a small, but real, possibility of increasing stroke risk as an adult. Please discuss these issues with the child's cardiologist.
Right heart damage. The ASD causes a chronic stress on the right side of the heart since it must pump the extra blood that comes through the hole. Over many years, the function of the right ventricle can decrease which may be permanent. Typically an ASD is repaired before 5 years of age to avoid this problem.