What could happen if asthma is left untreated?

Death, debility. Untreated asthma could lead to sudden death, although usually a prolonged period of gradual worsening of airway obstruction leading to changes in the lung that cause permanent airway obstruction. Worsening quality of life and the inability to do simple things follows... And an untimely death is always nearby. Get treated!
Varies. Asthma intensity ranges from so mild some may not know they have it & never seek treatment. At the other end of the spectrum are those that are hospitalized frequently for the disease & can rarely function off meds. In spite of great progress in treatment in the past decades, the incidence of sudden asthma death has remained relatively constant. If bad & untreated you die.
Worsening symptoms. Untreated asthma can worsen to the point that you end up on a ventillator or even die. I've personally disconnected the ventillator after declaring brain death for a child with out of control asthma. It was a very sad day.