Can asthma with 15 % of deficit of breathing get worse without treatment? I'm taking symbicort, (budesonide and formoterol) I'm healthy, but don't like to take meds..

Yes.... If airway inflammation is allowed to continue untreated, the disease can worsen over time. So, you need to take your medicine today to avoid more serious sequelae later.
Yes. At its base, asthma is an inflammatory problem of the lungs, & if you don't stay on your symbicort, (budesonide and formoterol) which contains an inhaled steroid to control the inflammation, it will get worse.